Lots of videos on Youtube of people making their own power hammers, but many of them seem to be just cobbled together with likely very little knowledge and other than the obvious non-stated "crank makes hammer go up and down and hot metal goes in here to squash it" I hadn't previously seen a detailed explanation of how they work etc.
I figured there must be a bit more to it than that....and there is....

Here's a link to Dave Manzer's "Powerhammer Cycle - the bang, tap, miss hit blues" that had previously been released on DVD.
https://youtu.be/ZECilDaxSes?

One of the best technical/instructional videos I've seen in a while.

Apologies if its been posted before, but I couldn't find a reference to it....

Steve