[vinhboy]

what exactly is that little dirt stain that moves from cylinder to cylinder supposed to be? is it supposed to represent tension or something?

[tptacek]

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.

[TallboyOne]

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?

[tptacek]

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.

[RandallBrown]

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.

[SquareWheel]

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.

[hoov]

Manufacturing tolerances aren't 100% perfect.

[jasonm23]

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