The 12s betting strategy in roulette involves placing bets on the first and second 12s (1-12 and 13-24). If you win, you take the winnings and place them on the remaining third 12 (25-36) and the second 12. You continue this pattern, always betting on the two 12s that you did not win on in the previous round. The idea is to cover more numbers and increase your chances of winning in subsequent spins.