NCAA When does a scrimmage kick end
 

Referencing the Memphis punt return TD against Miss St.

Miss St punts, ball goes over return mans head, it saved from going into the endzone at the 2 yard line by a Miss St player, who bats is back to the 4 yard line. Ball is grabbed, then dropped by a Miss St player and rolls until it becomes motionless at 4 yard line (temporarily motionless, maybe a second at most). The official throws the beanbag and his hands are starting to signal stop the clock, but the whistle had not blown. Memphis player runs in, scoops up the ball and returns the punt for a 94 yard TD.

The announcers were talking about how Miss St stopped playing and it shouldn't have counted.

what I do know, is no matter what happened on the punt return, had he fumbled or loss possession, due to first touching by the kicking team, Memphis would retain possession of the ball.

The question is when, by rule, does the ball become dead on a scrimmage kick? I thought it was when the ball becomes motionless on the field of play. If I am correct in that thinking, how long does it need to be motionless?

No matter what, the fact the Miss St players just stood around might have been the second dumbest play I saw today (Colorado St not picking up a fumble was the dumbest).

I think here is the play in question.

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Peace

The word "grabbed" in your description to me would indicate possession by the kicking team player, hence a dead ball. If the video posted by JRutledge is the same play, I wouldn't use the word "grabbed" to describe what I see.

There is one type of event that could supersede the illegal touch violation, and that would be the acceptance of a penalty for a subsequent foul by the receiving team during the down.

There is no answer to "how long does it need to remain motionless" in terms of time. It's a matter of whether it's motionless at a time while no player is trying to play it. The whistle doesn't have to be blown to kill the play, although obviously if one is blown that would kill the play if the ball wasn't dead already. If the official decided in his mind that the ball was dead, that's when it was dead.

If an official signals visually to stop the clock, maybe someone else can answer as to whether that's definitive evidence of the ball's being dead. There are more plays than this that could hinge on whether players should know that a ball is dead. For instance, instead of picking up the ball, suppose the Memphis player had blocked one of those members of the kicking team.

Same play happened in a HS game in my neck of the woods many years ago. Two local powerhouse teams. On a punt the ball was touched but not possessed. Kicking team started off the field. Receiving team picked up the ball and ran for a TD. (Receiving team player checked in with the referee to let him know the were going to try it.)

From what I've read in follow-up articles, the Memphis play was by rule correct, but the officials blew it as one on them was signaling "time" but hadn't blown his whistle.