As far as I can tell PoH is done by hashing the same value over and over (where each hash is hashing the last hash), to prove you've done some amount of work on

As far as I can tell PoH is done by hashing the same value over and over (where each hash is hashing the last hash), to prove you've done some amount of work on that block. So blocks contain both the hash of the previous block and a PoH hash which is showing you've iterated SHA256 n times. This way you can't spam the network with blocks. It seems similar to how NANO works. I still don't know why it's called proof of history.