OK, I've 28 years riding experience, but all on shaft drive bikes, so I'm a chain drive virgin. I was playing around with the rear sprocket while cleaning the chain. While fiddling with it, I rocked the sprocket back and forth by hand and noticed it seems to have a bit of free-wheel in it. That is, it appears as if the sprocket rotates separate from the rest of the hub, as the hub did not turn when I rocked the sprocket back and forth. What is the reason for this?