This seems like something that should have an easy answer, but I can't find it...
Suppose Tachyon has Unrelenting Momentum in play.
Action 1: She sprints, gaining a free Nimble Strike.
Action 2: She sprints again.
Unrelenting Momentum now grants Tachyon a free sprint. Also, Accelerated Assault grants her a free Nimble Strike. Do I have to choose? Or does she take both actions immediately on gaining them?
Here's a variation that really boils down to a second question:
Action 1: Tachyon sprints, gaining a free Nimble Strike.
Action 2: She sprints again, this time not taking the free attack, but Unrelenting Momentum grants her a free sprint. She sprints, and this time DOES take the free Nimble Strike. Does Unrelenting Momentum now grant her a free Nimble Strike? In more general terms, does Unrelenting Momentum apply multiple times per turn?
It triggers the first time you use an action for the second time but you have to use it immediately. So you could Sprint - Nimble Strike and then Sprint. At this point you have to use the free sprint or lose it. You can still Nimble Strike again due to Accelerated Assault. Since you didn't use an action directly for Nimble Strike you do not get an extra a Nimble strike from Unrelenting Momentum. Due to the extra sprint that you did you could make four Nimble Strikes due to Accelerated Assault.
Crush your enemies, drive them before you, and laminate their women! - Guise, Prime Wardens #31
The Nimble Strike is always free after a sprint, regardless of where the sprint came from (Ongoing effects are always on). So, if she sprints and attacks, then sprints without attacking, Unrelenting Momentum gives her a free sprint now, which, because of Accelerated Assault, gives her a free Nimble Strike afterwards.
Since Tachyon didn't use an action for the Nimble Strikes, they don't trigger Unrelenting Momentum. If she sprints, then Nimble Strikes (for free), then Nimble Strikes again (without sprinting), she doesn't get another Nimble Strike. She can, however, sprint 0 and Nimble Strike, thereby getting another sprint (which can be zero, and Accelerated Assault gives her another Nimble Strike. Or she can perform another Nimble Strike without the sprint and get a free Nimble Strike afterwards from Unrelenting Momentum.
"Deja-fu? You've heard of that?"
- Lu Tze, Sweeper, Thief of Time by Terry Pratchett
Can't you sprint/nimble the second time and still get a free sprint/nimble out of Unrelenting, or do I understand it wrong?
Hmm. I see. It wasn't clear to me that Unrelenting Momentum only applied when you use up an action point.
Yes. You can sprint/nimble, then sprint/nimble, then get the third sprint/nimble for free. I was trying to cover the unusual cases and didn't cover the normal case first. :D
Edit: Or you can sprint, sprint, sprint/nimble for free.
"Deja-fu? You've heard of that?"
- Lu Tze, Sweeper, Thief of Time by Terry Pratchett
...but you can't sprint, sprint/nimble, then sprint/nimble/nimble for free, because none of the Nimble Strikes ever were "taking an action" as I now understand it.
You can, but it's really sprint, sprint/nimble, sprint/nimble (free from previous sprint series), nimble
"Deja-fu? You've heard of that?"
- Lu Tze, Sweeper, Thief of Time by Terry Pratchett
You can Sprint/Nimble, Nimble, Nimble, Nimble as well.
But that's a lot of nimbling and your mother wouldn't approve of such behavior.