It's just simple leverage, if you think about and place the arm straight then the resistance increases pretty dramatically over the ROM from extension to the contraction of the deltoid, so if there is 10 foot pounds of force at the beginning then as you get closer to horizontal the foot-pounds of force increases 300% pretty damn quick
its a good exercise if you press a lot.
if you start with your arm straight and bend the elbow as you raise the weight you can adjust the load so it remains consistent