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A new Roller Derby Referee Hand Signal application has just been released on Android containing videos of the hand signals. And the best bit it's totally free!
play.google.com/store/apps/details?id=rpc.rollerderby.referee&feature=search_result#?t=W251bGwsMSwyLDEsInJwYy5yb2xsZXJkZXJieS5yZWZlcmVlIl0.