Would have used it twice ironically in 2 consecutive games after not having it all season, however, teams called TOs. In first one we were actually unsure about the helmet coming off instigating the rule until a crew member on the radio reminded us. Didn't matter, though.
Be sure you read the rule carefully if you haven't enforced it yet. It has been a long time since the summer clinic and having it for the first time in the playoffs is NOT a good thing, especially if you miss something.