I've just watched Scott Manley's video SpaceX's Water Landing Reveals Rocket "Secrets" (or, What We Learned from CRS-16) (again) and he mentions the second stage "pusher" in the center of the view (screen shot below).
I've added a small GIF of the video clip he shows of the stages separating.
In the black-and-white clip, I can see the white bit of the pusher extending. I am assuming that the end of the pusher already extends all the way through the long vacuum nozzle and expansion chamber and in contact with the back of the combustion chamber even before the stages start to separate, and that this is just more of it being exposed to sunlight.
Question: Roughly how long is the pusher in totality? Once separation begins, how far does it extend to accelerate the separation, just a few centimeters for a hard push, or does it continue to extend for tends of cm or even a meter to gently accelerate the separation?