The Top Roll is one of the basic moves in armwrestling. It relies less on brute strength (like in the hook) and more on opening up the opponents hand and fingers in order to get a leverage advantage.
How to Top Roll
At go, apply backpressure to the opponents hand while simultaneously twisting your wrist/forearm towards their fingers. You should go from having the palm of your hand in a neutral position, to facing away from you. The point is to open up their hand (literally opening up their grip) and bending their wrist backwards into a deadwrist position. This helps to remove any power advantage they may have in the hook. If possible, walk your fingers higher up into their hand, which will increase your leverage and greatly decrease even further the power they can exert.
For pictures follow this link: http://www.eiyc.com/toproll.htm