How do you implement high risk/reward in combat?

        Through discussion, a loose conclusion was reached that most tactical combat tends to lean heavily towards hedging your bets, so to speak. Tactical combat seems inclined to take a low risk approach, when looking at the math. While I'm not saying this is wrong or right, I myself can be heard declaring we should "300 this" quite often, I'm wondering if there are systems or home brew rules that support a high risk/high reward combat style? The support for a desperate gamble if you will.
        If not how would you go about properly rewarding that desperate gamble moment where the outcome could be very bad or very beneficial? I'm kinda mulling the idea of pulling luck in to modify a roll, then have a success at what cost mechanic. I know, mechanically speaking, this is all out of context; I am kind of trying to bring an abstract idea into a more defined ... thing.
        The character jumps on the creature that would surely kill his downed friend, he manages to somehow slay something he should not have been able to, but now could bleed out from the wound received in this desperation maneuver.
        Hmm, a work in progress ... Also, a quick thanks to all the people who are constructively contributory, I really appreciate the feedback.