It's the currently binding cylinder. With the right amount of tension applied via the wrench, one pin binds at a time, which is what allows you to set the pins one at a time.

What's the difference in feeling where he's 'testing' which one's the binding cylinder or not? Is it just a tiny bit more 'grainy' harder to push or something?

The binding pin won't bounce up and down when you actuate it with the pick. The non-binding pins won't offer any resistance at all. Also, when you manage to get the binding pin to the shear line, the tension wrench will sometimes give up a little more rotation.

why does only one cylinder bind at a time?

It seems to me like the lock shouldn't be able to turn at all until all of the cylinders are pressed to the right height.

Due to imperfections in the lock. It only takes a tiny difference for that tension to not be applied evenly so you can focus on one pin at a time.

Manufacturing tolerances aren't 100% perfect.

it's the next 'key' pin. (the little stain is a key icon.)