For those of you that are still confused:

Bet + winnings – previous bets = initial bet

2+2-2=2
4+4-4-2=2
8+8-8-4-2=2
16+16-16-8-4-2=2

The idea is that if you bet something like $100, you can guarantee earning $100 if you can continue to fund doubling your bet until you win.