system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How does one end up fainting due to locking their knees?
gpt
Locking your knees has the effect of stopping your calf muscles from needing to move to keep you upright.  You’re supported by your bones instead, and kind of “hang” off your skeleton.

This doesn’t seem very important, but your blood actually relies on your calf muscles to get back up to your heart.

Veins are the blood vessels that flow back towards your heart.   They are low pressure, unlike arteries which flow away from your heart and are high pressure.   Because of this, and because they go against gravity, veins have valves inside them to keep blood moving in the right direction.   Veins are found between your muscles, and every time you move the muscles squeeze the veins, which pushes the blood up through the valves towards the heart.   

When you lock your knees, this stops happening,   Your heart keeps beating, and keeps pushing blood around - down towards your legs - but your legs will begin to form a pool of blood.   Less blood being returned to your heart means that there is less blood available to go to your brain.

Now, your brain *needs* blood constantly.   Like, way more than any other part of you.   When it doesn’t have enough, it panics.   It has exactly one cure-all insta-fix for not getting enough blood, and that is to force you to instantly put your head level with or below your heart.   It’s hard to get blood up against gravity, much much easier to do it on a level plain, and so it’s easier to restore blood flow to your brain if you’re lying down.   So your brain makes you lie down, and makes it so that you have zero choice and zero time to delay - you faint.   

That is how locking your knees leads to fainting.