When you attack, you may attempt a "normal attack," a "simple stunt" or a "combination."
Normal Attack: This is executed according to the rules as written. Roll your d20, apply modifiers. If you hit, you do normal damage.
Simple Stunt: This is executed as a normal attack. However, in place of doing normal damage, a successful attack results in a non-damage effect that you declare (push an enemy back, trip, disarm, etc.).
Combination: A combination must be declared prior to making your attack roll. You then make your attack roll as normal. If your modified d20 result is an even number AND is sufficient to hit the target, you succeed and get to choose between the following two options:
1) inflict double damage OR
2) inflict normal damage plus a stunt effect
If your modified result is either odd OR if the result is not enough to hit, your entire attack/stunt fails and has no effect whatsoever. In effect, a combination cuts your chance of success in half, while doubling the effect if you succeed.
Nit-Picky Stunt Details
Stunt Effects: All references to "stunt effects" refer both to simple stunts and to the stunt portion of a combination. Common stunt effects include things like tripping, pushing an enemy back, or disarming. However, there is no definitive list, and stunt effects can be any clever effect or maneuver you come up with, subject to GM approval or limitation. Stunt effects can even apply to missile attacks – for example, shooting to one side of the enemy to "push" him the opposite way.
Short Duration Stunt Effects: if your stunt effect is something that would have a short duration (e.g. stun, knock-down, disarm, etc.), the opponent will typically recover in 1d3 rounds. This can vary by circumstance. Some stunts may cause an "immediate" effect that doesn't last beyond the current round (e.g. pushing an enemy back).
Long Duration Stunt Effects: if your stunt effect is something that would last all the way to the end of the combat or longer (blinding an enemy, disabling a body part, etc.) the target of the stunt will generally be allowed a saving throw to negate the effect.
Declaring Stunts and Combinations: You must declare a stunt or combination, and its desired effect, prior to your attack roll. Failing to declare causes your attack to default to a "normal attack." Once you declare a stunt or combination, the GM will decide whether to allow the effect, and whether to apply some limitation to it. You can then accept the limitation (if any), renegotiate the stunt effect, or switch to a normal attack, prior to rolling the die.