Watch the Tim Stafford Style Rhythm online guitar lesson by Stephen Mougin from Bluegrass Rhythm Evolution
Sometimes when Tim Stafford approaches uptempo bluegrass numbers, he'll put a hard stop in his strum. It appears to be placed preceding the downbeat of a new section, or something he's trying to draw the listener's attention to. It's a very percussive sound, and works wonders for the drive of the ensemble.
(User listening tip: "Last Dollar Blues")