Here's an even better explanation from p. 60-61 of
Football Rule Differences 2007: NFHS & NCAA Rules Comparedby Paul Whiteside, George Demetriou, and Jeffrey Stern:
The game clock starts on the snap whenever team B is awarded a new series; however, there are different definitions for when a new series is awarded.NFHS:A new series is not awarded to team B until all acts that occurred during the down, including penalties other than nonplayer or unsportsmanlike, are considered (3-4-3b, 5-1-2b)
Play 1:Second and five on team A's 35 yardline. B2 intercepts A1's forward pass and is tackled inbounds on team B's 42 yardline. While the pass was in flight, B2 committed pass interference and the penalty is accepted.
Ruling 1: NFHS: The game clock starts on the ready. Team B was not awarded a new series because of the penalty.