You are a creature.

You need to puke at enemies to kill them.

If they hurt you, your legs will drop.

If you hurt them, they will drop their legs.

If you are missing legs, you can take fallen enemy legs.

If you are low on health, you can eat your own legs to heal.

AD - Left and right

W - Jump.

(Arrow keys also work)

Space - Eat a leg to heal.

Left or right Shift - shoot pukes.

However, the less legs you have, the lower you jump and the slower you walk. That is also correct to enemies with legs.