To lock the throttle. Never used it on any of my bikes.

 

It is a great device for use on the long runs on the interstates. As you tighten it, it puts friction on the throttle. You can adjust how much tension is on the throttle to allow you to make adjustments on how stiff the control is. As you play with it you will find a sweet spot that keeps it at a constant speed, yet allows you to quickly pull back throttle to slow down.
Like stated above, a poor mans cruise control.

 

You can hold the throttle wide open and then let the clutch fly at the drag strip, its like a poor man's stall converter (:

 

Mine has a little aftermarket lever that the previous owner installed over the ****. But it's not working after I changed my bars yesterday. I made sure the little plastic curved plate was still in there. I think I just need to take off the little lever, adjust the ****, then put the lever back on.

 

Mine doesn't do anything. Just spins.

 

Yours is stripped.

 

So just if anyone was wondering, my throttle lock has a little snap on lever. So when I changed to a different throttle tube, the diameter is smaller of this tube, so the lever wasn't pushing the curved plate in far enough to touch the throttle tube.

Simple solution: Popped off the plastic lever cap, adjusted the star **** until it was just free of the throttle tube, and popped the lever cap back on.

I don't know how much these lever caps are, but I recommend one. You push it forward with your thumb, push it back with your index finger. Makes throttle locking super easy, and in my opinion, safer to use when riding.