It's not that well known, but the Limit Breaks from Final Fantasy VII were based on a hidden mechanic from the previous game. When a character's health was less than 1/8 maximum, there was a 1 in 16 chance that choosing to attack as normal would instead unleash a hidden ability unique to each.

See, for example, Setzer's Red Card (Bloody Card in the original Japanese). The freewheeling gambler deals himself a hand or cards, touches them to his scarred face, and sends them flying with a whispered imprecation. The cards glow red and strike the enemy with a spray of cursed blood, bypassing their physical defenses and dealing magic damage.

Credit where credit is due: the background is from the game itself (the upper deck of Setzer's beloved airship, the Blackjack); character and game are (c) Squaresoft/Square-Enix.