The video shows just one of the possible leveraging feats that can be done with a sledgehammer. This one involves lowering the weight to the nose then pulling it back up. Always a crowd pleaser, because it involves the danger element. If you were to not have the strength, the hammer would come down hard.
Other levering feats include raising the hammer from the ground to an upright position or just holding it out straight from the end.
All these moves involve the hand movements called ulnar and radial deviation. That is bending the hand from side to side, an oft neglected degree of movement and strengthening that is important for many feats like bending steel. In addition you need a strong grip to hold onto the handle.
Slim ‘The Hammerman’ Farman is by far the strongest in all these moves, hence the nick-name. His records are likely never to be broken. Here’s a small clip of him in action.