Motor/Servo Clutch
From VEX Wiki
A VEX clutch is made up of two main parts. The main body of the clutch, is one part. Inside this body rotates a second part. Both of these parts have a socket in them that will accommodate a Vex shaft.In the standard (pre-packaged) configuration one of these sockets has a short piece of shaft in it, which then sticks into the Vex motor or servo. The other socket in the clutch is left open, and the user can insert any Vex shaft.
When a clutch "pops" the inside cylinder will slip from the outside cylinder, and as such the two sockets will rotate seperately (or one will remain stationary, while the other rotates).
If a clutch is functioning properly, you should hear a popping/clicking sound as significant shock load is applied.