Combat base is a specific posture used in half guard passing and closed guard passing. With one knee up and one knee down, it creates a stable base that resists sweeps while providing passing options and submission threats.
From inside someone's half guard or closed guard, establish combat base by placing one foot flat on the mat (knee up) and kneeling on the other knee. Your weight sits centered between both legs.
The combat base counters many guard sweeps because it spreads your weight over a wider base. The elevated knee prevents them from inserting their knee shield effectively.
From combat base, you can attack kimura and darce chokes by controlling their near arm. The position also sets up guard passes by loading weight toward their legs.
When you stand to break closed guard, dropping back to combat base (instead of fully standing) maintains guard control while allowing posture recovery and arm attacks.
Transition from combat base to a full guard pass by driving your knee-up knee toward the mat and using a knee slice or leg drag entry.
See also: Half Guard Top Guide, Technical Standup Guide, Guard Break Guide, Posture in Guard