Any vibration is wasted energy. The dead blow conserves that energy by not letting it deflect back, the shot hitting the inside of the chamber at just the moment that the energy would be reflected.

That is why Harleys are so slow, they are the worst possible engine one could design, a 45 degree V with both rods on the same journal. Whenever you see or feel a vibration, that took energy to create, and so is wasting energy that could be used for harnessed motion. In the case of the Harley if you manage to run it up to enough RPM it will literally destroy itself through its own terrible primary balance.

The dead blow hammer is the answer to the wasted energy used to move your arm that is lost in vibration.

I use several myself but the nicest one and my favorite is the one I bought from the SnapOn man. Very good balance.