A gag works by lifting the bit in the mouth in order to encourage the
head to be raised. Often used on strong horses that like to lower their
heads in order to escape the action of the bit in the mouth and get
very strong.

When pressure is applied to the rein, the bit is
lifted in the mouth and poll pressure is applied. The bit also rotates
slightly in the mouth putting more pressure on the tongue and bars.

This is a very popular bit on the Polo field.

To be correctly used, it must be used with two reins one on the gag rein
and one on the bit, only bringing the gag rein into play when needed,
although most people ride on the gag rein only.