You need to set the power at the correct pins (which basically is the input) and after the electricity has gone trough the hardware you are measuring which bits are set (output; effectively the hash).
The time it needs to calculate a hash is the time the electricity needs to run through the hardware (not exactly, but basically).
Yeah, so currently a double hash is like 2000+ sequential operations, so there's a maximum frequency you can operate while you wait for the output. If you somehow could use just 1000 gates, you could run it ~twice as fast, right? Or is there some other limitation?