The hammer/plate has been around for a while. It is simply easier to break a plate with a hammer. The idea of the minhag is that both mothers are doing it together, so what is the difference if they both hold it and smash it (risking injury, btw), or one holds and the other hammers (also risking injury)…80
I don’t know if there is a difference, I just never heard of it before…80
In our chasunahs, the two machetenistas held the plate and smashed it simultaneously, but many people are doing it the other way, buying specialty hammers for the pupose (decorated beautifully, etc.). It comes in a kit with the plate, a pretty holder, and the hammer.