system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How a dyno determines how much horsepower a car has from the tires spinning.

Is it how fast the drum takes to get up to speed? I'm sure there is alot more to it like gearing, but this has always boggled me.
gpt
The simplest type uses a(very large)  weighted wheel of a known weight, and the engine's ability to accelerate that weight to determine power. That's how a "mustang dyno works. Other dynos use a known load, either an amount of water pumped, or an amount of electricity generated to find out the amount of power created. The mechanisms behind both methods are much more complicated, but are generally accepted as better, and more useful results, since they can be applied "steady state". Meaning an engineer can find out how much power is produced at any given rpm continuously.