These operate in a similar premise to the framing hammers you see with the waffle face. On those, a smooth hammer face would often result in a deflection or bending of the nail depending on how inaccurately the hammeree was swinging. The theory is that the waffle face gives you multiples of "small hammer faces" where if one slips there is another nearby to grab the metal being swung at for a more effective hammer strike.
Having said that, the shrinking hammer shouldn't be operated as a sledge hammer, you are NOT trying to transfer the waffle pattern onto the metal surface to "gather up" the stretch. Light to moderate taps are more effective in providing the striking force without any "slippage" like a smooth hammer may experience. More effective on something that is not so smooth, like if it resembles a walnut shell, as it will tend to help push down the high spots, and it's "multiple hammer faces" tend to grab the next high spot as the first is being pushed in.
A gimmick hammer? Of sorts, yes. Used incorrectly they can increase the damage. It's that fine line between almost there and now it's stretched even more.. But when this hammer was first invented tools like shrinking discs did not exist. Now these new tools are more widely used as they are less finicky in producing results... As most are indicating, they are seldom used. So if you want one, a used one would be the way to go. That way you didn't pay full price for one to sit in your hammer drawer as well..
