system
Given the following post, respond as if you're commenting on Reddit:
human
What exactly does it mean to "lock" your legs and why does it cause people to faint or pass out while standing?
gpt
It means your knees are bent slightly backwards, so that your thigh and your shin are closer to each other in the front than in the back. 

The movement of your leg muscles is part of what brings your blood back from the legs to the heart. This is called the muscle pump. 

Locking your knees means you aren’t moving your legs, which means no muscle pump — and in absence of muscle pump, your blood pressure is lower than usual.

Low blood pressure can cause low oxygen to your brain, which results in you passing out.