Inspired by this comment from Lasgunpacker over at Jeff’s Gameblog, here is a simple rule for combat stunts.
Say you want to perform a combat stunt that is high risk but also high reward (such as doing a backflip over charging goblins and kicking them over a ledge). The referee will give you a difficulty modifier based on the specific circumstances (say, -2) and if you accept the risk you make an attack roll. If you hit, your stunt succeeds (exact effect is by referee ruling). If you miss, it counts as a fumble, as if you had rolled a natural 1. Either roll on a fumble table or do whatever it is your group does for fumbles.
Mathematically, this compresses the probability distribution. Everything above the target number is something like a critical hit, and everything below is a fumble. It is the combat equivalent of “all in” which captures exactly the flavor that I think a stunt should have. Also note that a penalty to the attack roll need not always apply as the increased probability of a fumble may sometimes be enough of a difference from a normal attack.
Many systems use auto-success mechanics for stunts (e.g., luck or action points) which create exactly the opposite of the desired narrative effect: certainty rather than suspense. Another common game mechanic is to call for something like a dexterity or athletics check. I also consider this suboptimal because it emphasizes ability scores and thus character engineering. My dislike of emphasizing ability scores should by now be well established.