A hackamore is a bitless bridle that uses pressure/leverage on the nose, chin and behind the horses ears. 
The hackamore, also known as the hackamore bit comes with different shanks. The longer the shanks the more leverage it has.

The hackamore bridle is very popular due to its quick reaction and its direct release meaning that when no pressure is given with the reins the pressure immediately disappears. 

The hackamore is not deisgned to give constant pressure or to be ridden with with constant contact and is made to help support your body language when ridig.