Make yourself ready by putting your weight on the balls of your feet and making two tight fists. Guard yourself and avoid any incoming blows while looking for your opening.
Your opportunity arrives when your opponent's midsection is unguarded and especially when he is exhaling. You must act quickly and without hesitation, if he knows what's coming he may tighten his stomach muscles and negate the effects of the blow.
The best time to strike is right after your opponent has swung at you and missed. get close enough so that you could punch through where your opponent is and to a point behind him if he were not there.
In the same motion as lunging toward your opponent, aim a punch at the space just below his ribs on the centerline of his body.
Also in the same motion, deliver the punch with as much force as you can in an upward motion. Pretend you are trying to push his stomach up into his chest.
Continue the upward motion of the punch until your opponent gasps and hunches over, then withdraw your fist quickly. If your opponent is struggling for breath you have created your escape opportunity. Run! You have between five and twenty seconds to get away or get help.