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.
When you turn a lock, small machining defects mean that more load is borne by one pin than the others. Pushing up that pin while applying torque (using the allen wrench in the GIF) will cause that pin to be held up, which applies more load to some other pin.
Here's a version I found on reddit a while back.