When you are sitting in someone’s closed guard you have one objective: Clear their legs while not getting swept or submitted. The ideal starting position is one where you can be safe from the guard player’s attacks and are in good position to open the guard. Also when strikes are involved you want to place yourself in a position to strike effectively.
What is proper jiu jitsu posture?
Jiu jitsu posture is different from regular standing posture because there are different objectives. In jiu jitsu when you are sitting in someone’s closed guard you are mainly resisting a forward pull from the guard player. To remain balanced you have to keep your center of mass between your base of support, which in this case is your knees and feet. If the guard player manages to pull your center of mass outside your base of support you will be unbalanced. When unbalanced you will be susceptible to sweeps, submissions and it will make passing much more difficult. Proper posture is essential to keep yourself safe and to be able to attack effectively.
Initial posture: Tuck your tailbone underneath you (posterior pelvic tilt). When your pelvis tilts posteriorly it causes your spine to have slight flexion. The spinal and pelvic alignment puts you in a mechanical advantage to resist the guard player’s anterior pulling force. Why? If you have neutral spinal alignment and they pull then you will be pulled into lumbar extension ie your center of mass will be much closer to the edge of your base of support. Therefore the force they will need to unbalance you will be considerably less.
The most stable alignment is a neutral spinal with abdominal bracing, however if there is slight flexion it will not significantly decrease your stability.
When the opponent’s pull increases or if they try a hip heist sweep to unbalance you posteriorly you can “engage your hips” meaning activating your glutes and slightly lifting your hips up to meet their increased force.