So I've been hearing more talk about folks being confused about how Dimensional Instability works, and since it's basically the most complicated rule in the game (there's a reason it has five FAQ entries lol) I figured I'd summarize everything here for anyone who's feeling confused about it. I'll throw in some tips and tricks throughout too to help you all think with portals.
To start, let's go over the actual wording of this ability, nicely colour-coded to split it up into its three parts:
Dimensional Instability: During Scouting, place three 50mm Portal Markers on the board in any location at least 8" from any other Markers and not in your opponent's deployment zone. When a Cult of the Burning Man Fireteam in this Company comes into (or starts in) base contact with a friendly Portal Marker, you may place its unit in base contact with any other friendly Portal Marker. Fireteams that move in this way may not use any remaining movement that was granted by the Order, and cannot use the Dimensional Instability rule again during this Activation.
The First Sentence
This part's simple: when you're setting up the board, you get to put down your Portal Markers in the Scouting step, specifically in step C. Company Abilities (after any fixed position Objective Markers go down and before Objective Markers that you and your opponent choose the locations of go down).
The Second Sentence
This part's a bit more complicated, as it needs you to really understand how place effects work, so let's start this off by going over the important bits about places and Portal Markers from the rulebook and FAQ:
Place: Place effects are a special form of movement in The Other Side. When an object is placed, it is picked up and placed in a specific location, as determined by the text of the effect creating the place. If the place has a range, the object must be entirely inside the place range. If an object does not fit in a location, it cannot be placed and does not move. When an object is placed, it counts as having moved. A Fireteam may not be placed on top of another Fireteam.
27. If a Fireteam uses the Dimensional Instability Ability to leave the engagement range of an enemy Fireteam, does it suffer a Hit for disengaging?
29. What does “starts in” refer to on the Dimensional Instability Ability?
a. “Starts in” refers to a unit starting its Activation in base contact with a Portal
30. Can a Fireteam use the Dimensional Instability Ability from any instance of base contact with a Portal, such as Tears in Reality or a unit of Stalking Portals moving into base contact with them?
31. Can a Cult of the Burning Man Fireteam use the Dimensional Instability Ability during deployment if it is deployed in base contact with a Portal?
With all of that in mind, let’s review what's going on. Let's say that some single-Fireteam unit such as a Breachling wants to go through a Portal Marker, but they’re not in base contact. The Breachling can move over to the Portal Marker and use Dimensional Instability as soon as it comes into base contact, or the Portal Marker can itself be moved over to the Breachling for the same effect. Really all that matters is that base contact is achieved, how the base contact happens does not (even if the movement was caused by an enemy effect, such as Siren’s Call!), although you can’t use Dimensional Instability during deployment. As soon as the base contact happens, the Breachling can place into base contact with another friendly Portal Marker of its choice. If the unit is already in base contact with the Portal Marker when it activates, then it can just use Dimensional Instability right away without having to do anything else.
Okay, let's say instead of a Breachling it's our lovely Titan the Goryshche who wants to use Dimensional Instability. The Cult Titans are a bit of a special case, since they can move through non-Titan Fireteams (though they still can't end their movement overlapping another Fireteam), and because of that they have an extra FAQ just for them:
26. Can a Cult of the Burning Man Titan use the Dimensional Instability Ability while its base is overlapping another non-Titan Fireteam’s base?
a. Yes. However, when placed, its base cannot overlap any other Fireteam bases.
So if the Goryshche wants to go through a Portal Marker that an enemy non-Titan is standing on top of (partially or fully), Goryshche can just walk on top of the enemy and use Dimensional Instability even if where it came into base contact with the Portal Marker wouldn’t be a legal place to stop moving. It's a bit niche, but this makes Goryshche and Horomatangi a lot harder to stop from getting where they want to be.
So that's us covered when there's only one Fireteam to deal with, but how does being a Squad change things? Well, there's a bit more from the rulebook and FAQ on that:
Interaction with Squads: When a Fireteam from a Squad is affected by a place effect, every Fireteam in the Squad is placed unless the effect specifically states otherwise (such as by saying “one Fireteam” or “only the targeted Fireteam”). When multiple Fireteams are placed at once, as many Fireteams as possible must be placed in the indicated position. Any Fireteams that cannot fit are then placed in base contact with one of the Fireteams that was able to fit in the indicated position, in a position determined by the Fireteams’ controller. Placed Fireteams must end the place effect in Formation, as usual.
3. When placing a unit, is every Fireteam considered to be placed at once or are they placed one at a time?
a. Every Fireteam is considered to be placed at once and must be done in a way so that as many Fireteams follow the initial placement effect as possible.
What does that all mean? Well, there’s a few big things. First, all of the Fireteams must place into base contact with the same Portal Marker, no matter how far away they all were (sorry ECB Black Ops). Second, only one of the Fireteams in the unit needs to be touching the Portal Marker. But, third, you need to place as many of the Fireteams into base contact with the Portal Marker as possible when you’re placing them. Any Fireteams that absolutely can’t touch the Portal Marker are instead placed into base contact with one of their Squad-mates who did manage to get into base contact with the Portal Marker.
Because of all of this, if your landing spot is a bit tight you can still conga-line your units in a weird way to make it work. Let's say you have three Fireteams in the unit and only one of them can touch the Portal Marker. If only one of the remaining two can touch that first Fireteam, then the third Fireteam can be placed in base contact with the second one. You just have to place them all as legally as possible, but the downside to this flexibility is that if you can't work it so all three fit in some way or another then NONE of them are placed. In general though, conga-lining your Squads is something you can intentionally work to your advantage: if you have a friendly model place itself by your second Portal Marker in such a way that a Squad that lands there would legally have to conga-line which can give you a bit more distance if you set it up right.
The Third Sentence
This is another section that gets a lot of questions, specifically about how using movement that wasn't from an Order affects things. To help explain this, let's go over one last FAQ:
28. If a Fireteam moves through a Portal using the Dimensional Instability Ability from an effect other than one generated by an Order, can it still resolve movement generated from an Order during the same Activation?
So there are two things to keep in mind here: first, if the movement you used to trigger Dimensional Instability was from an Order, as per Dimensional Instability's blurb you cannot use any remaining movement from the Order after placing. Second, if you trigger Dimensional Instability either by starting in base contact with the Portal Marker or by using non-Order movement during your activation to get there, you still do not get to use any movement from your Order for the rest of the activation. However, that is only true of movement that's from an Order – if you have other ways of getting movement, such as from Goryshche’s Splitting Tails, you absolutely can still use that movement after placing!
Also, if you used Dimensional Instability outside of your unit’s activation, such as something moving you outside your activation into base contact with the Portal Marker, when your unit activates then it can use any movement from its Order (as long as you don’t do Dimensional Instability again). This is one of the big reasons why Adeodatos’ Twisting Paths and the Doomseekers’ Translocation Ritual are super handy for moving folks around in unexpected ways, as they give you a lot of extra movement if you play them right.
Alrighty, and that's that! I hope that helped make a little more sense out of the most complex (and confusing) ability in the game. If anyone has any more questions about how things work, or have any tips and tricks you want to point out, please post them below and I'll happily edit this to add them in!