HLin NC nailed it for NCAA Rules. If the guy goes out of his own accord (or doesn't come back in after being blocked out) then he loses his eligibility to touch the pass until the D or an official touches it.
Mechanics?
Some will drop their hat at the spot he goes out (sometimes called the "Honig mechanic", the thought being Honig's get to sell more hats that way to replace ones that get trodden into the mud
). Others will beanbag the spot.