MrTheEngineer's write up on horsepower reminded my of how I was always confused on how exactly Dynanometers measured HP until I looked up their history, specifically the de Prony Brake (ie why we call it Brake Horse Power)

This video on mrpete222's Youtube channel shows a de Prony in action, he does a great job explaining the process as he goes along.

Part 1 of the video goes into more detail including different formulas for different length brakes. It's also easy to glean why the industry has moved away from this method.. as it would be pretty fucking dangerous on modern high output motors at higher rpm.