To swing a massive hammer like that, he'd need at least some momentum - it's hardly an arm strength thing, it's more about how much energy you build up in your stance before swinging.
That said, imagine swinging a massive sledgehammer. When I do it in the mirror, I crouch a little and bring the hammer back behind me, bringing most or all of the weight of my body on the back leg, bending it. Then i use all my upper body strength to swing the hammer over my head and at the same time straighten my back leg, gradually bringing my weight to the front with the hammer. To finish off I'd either end up dragging the hammer back on the floor after impact or if i'm swinging it in horizontally, it would end up behind me and i'd swing it back to its' original spot slowly.