I think they're doing a prize every night at 7.00 pm - and it's either drawn from everyone who is logged on to Rocks at the time or everyone who has played the game and submitted their score, which from what was said the other night I don't think it has to be high enough to be on the leader board for you to be entered, so just because some people are ace and get really high scores doesn't mean that they will be the one to win the prize. I think that's how it works.
ETA: the last two nights they've tossed a coin to see whether the prize is drawn from those logged on or those who have played the game.