OH CRAP - you asked about short kick mechanics. Sorry. I misread. Read below for deep kicks, but I'll follow up for Short Kicks.
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Well, we know the F and S have goal line. They need to hustle, stay ahead of the BC, and beat him to the goal line, if at all possible, and be holding still on the GL when the BC gets to or crosses the GL. They have progress between the A-2 and the A GL.
Now, as for the B and U, the manual only says for them to have primary responsibility for action toward the middle of the field, since they are at/near the hash marks. They should flow with the action toward A's goal line.
I recall back in my FBS days, that we wanted the B to hustle and also beat the BC to the GL, to 'bracket' the BC between himself and F or S on the goal line. We don't seem to have that in our TASO manual. We still have the H, L, and R trailing the play to observe action behind the POA, in addition to the B and U. I would hope that the B would maneuver himself to be able to assist at the GL, if needed. Maybe not ON the GL, but near enough to offer some assistance to the wings, especially if the BC crosses the GL between the hash marks.