This is the moving part on which the rope rotates.
It is a fixing point under the pulley to provide a starting point for the rope that forms the hoist.
A violin pulley, is a pulley with a second, smaller sheave under the main sheave. This configuration allows to make a multi-strand hoist without the double pulley.
The clamp blocks the rope at the end of the pulley. This is very useful for keeping a mainsail sheet in position. The inclination of the clamp is adjustable to make the lock/unlock operation as easy as possible depending on the position of the adjuster.