To rest a part of one's body on top of s.t. else (as when resting one's leg on s.o. or s.t., leaning one's upper body over a desk, etc.).
The -an imp. focuses on the place where one is resting a part of his body. The pa--un imp. focuses on the part of the body that is resting on s.t.
Don't rest on your younger brother's thigh.