[rule] Scenario C2.2.1.B
Before the Jam-Starting Whistle, Red Jammer skates backward. They come to a quick stop and shoot forward at the Jam-Starting Whistle.
Outcome: If Red Jammer was moving clockwise or stopped when the whistle occurred, nothing should be done. If Red Jammer turned counterclockwise earlier and was gaining speed, this should be treated as a false start.
Rationale: Jammers may not be gaining speed counterclockwise at the Jam-Starting Whistle.
[/rule]
So not a penalty but a warning (if they accelerate *at* the whistle - if they accelerate before the whistle and coast at the whistle that is legal). And as the scenario indicates, the verbal cue "False Start" (without hand Signal - see cues codes and signals document) is suitable for the situation.