The Allen ratchet block has one purpose; to add friction to a control line system to make it easier to hold and adjust.
This is achieved with two features. Firstly, a sheave that can only spin in one direction. Secondly, by having a sheave which is shaped to grip the rope. The internal mechanism is where all the magic happens. To create a sheave that can turn in only one direction requires a toothed design paired with a spring-loaded pawl. As the sheave turns the pawl jumps over the teeth (this is where the clicking sound comes from). As soon as the load changes direction the pawl locks into the teeth which stops the sheave from turning.
Once the sheave is locked into place the outer profile comes into effect. Ratchet block sheaves are designed with a V shape, this is to create a gripping or jamming effect on the rope. This gripping effect combined with a sheave that only turns in one direction adds friction, which makes it easier for the user to adjust a line when under load.
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