The basic idea: when you reach 0 hit points, you're not dead, you're just unable to defend yourself and every hit is potentially lethal from that point on. If you're hit, you roll on a table (2d6) that has a variety of effects like "knocked down", "stunned", "limb chopped off", "mortal wound"… and if you roll 12, you get 1d4 hit points back until the end of cr0mbat!
Kinda cool. Could make combat more tense—instead of the 1-hit kill, there would likely be more injured survivors. And there'd also be a fair number of dudes missing arms and legs. :)
Vancouver Red Box: a site for old school D&D