It was the Martingale, a type of doubling-up gambling method for 50-50 systems. The idea is to have a base bet, and if you win, you bet that base bet again. If you lose, you double the base bet and bet again, knowing that in a 50-50 game the odds are good that a win will come soon, compensating for the loss and bringing a win. Each loss requires a doubled bet, which can cause disaster if the unlikely event of multiple losses in a row occurs.