Double after every loss; the first win recovers everything plus one unit. The claim is that you cannot lose. The math says you win almost every night, and the nights you lose pay for all of them, with interest. Here are the numbers.
Seven losses in a row and a $10 bet has become $640, with $1,270 already gone. Seven straight losses sounds rare; at blackjack it happens about once every 250 hands. Play a couple of nights a week and the ladder ends on you, monthly.
Table limits exist because of this system. The house does not fear the Martingale; it caps it so the cliff arrives before your bankroll can climb away.
Dealt the identical 100,000 hands as flat betting, the Martingale wagered three times the money and paid three times the house edge for it. It converted many losing nights into winning ones, and concentrated all the losses into a few nights nobody walks away from. The average is untouched. It always is.
Related: every betting system priced on the same cards · why fair blackjack feels rigged · the solved strategy chart