What is he trying to do? Throwing a bomb at himself?
If I were you, I'd act it out, physically. Try and see what your body does when you perform this action. Film it if you need to.
For me, what my body does:
First I extend, even on my right toe (when throwing with the right hand) to make myself as long as possible. The anticipation goes slow compared to the throw. Then as I pull down to the floor my chest turns, with my right shoulder going to my left foot. My upper back ends up completely rounded, my knees slightly bend. My head is turned away from the bomb, so to the left, as far as possible, with my left elbow covering my face.
If you would draw all of this motion, I'm sure he wouldn't look stiff anymore!