So basically rollout has two states after a charge, damaging and non-damaging.
after you press B, frames 1-15 are listed as animation 300. This is her little bounce before she starts charging on the ground.
frames 16-52 are listed as animation 302. This is her charging animation.
frames 60+ reads as animation 303. The fully charged sparkles don’t appear until frame 62, but rollout is considered fully charged from f60.
If you release B at any time between frames 1 and 35 then you will release into a non-damaging rollout. How long you held B will effect your velocity but the hitbox will have no effect.
HOWEVER, there is a tiny hitbox in puff’s center that appears in non-damaging rollout for 1 frame as soon as B is released (or on f16 if it was released prior to f16) and when puff completely changes direction (accompanied by a low whoosh sound effect). I can’t list on what frame the turning hitbox appears because how long it takes to turn around depends on your velocity. This hidden hitbox was probably a joke put in by the developers because it is so nonsensical and difficult to hit with. If it connects in the air the KB is very poor. If it connects on the ground then it has decent KB at an amusingly good angle and a fair amount of KB growth.
If you release B at any time between frames 35 and 59 then the normal rollout hitboxes will be active. You can tell if you’ve released during this range because you will sparkle as you roll. KB and velocity increases with the amount of time that you held B.
If you release B on frame 60+ then you will be fully charged.
On grassy surfaces (including the very edge of FoD and the grass transformation of PS) you will skip the skidding portion of turning around while in rollout. This allows you to reverse your direction an additional time.
I personally don't think that rollout has any meaningful utility outside of mayyyybe some weird situations on FD or a few rare situations involving a read on missed tech.