I want to bring that hand behind the leg, but it always bumps back into its default position.
How can I set the settings that this doesn't happen anymore?
If the current motion of the hand is controlled by a Timeline animation, then you would have to go through the timeline to all the keyframes on the hand and move it. It's either that, or remove that animation and replace it. It's not a default position, it's most likely a pose determined by the animation.