Dumb bell Bicep Curls

I noticed that when I am doing bicep curls with dumb bells if I finish the move when I lower the dumb bell and the palm of my hand is facing outwards seems to hit my biceps a lot harder than if the palm of my hands is facing towards me. What is the difference?
Years ago when I trained with body builders they always seemed to turn the wrist at the end of the move to where the palm is facing their bodies.