A thigh harness is a harness with a dildo someone can strap on the thigh.
It normally consists of a rectangular piece of material, with two fastening straps on either side. There is a round opening in the middle, in which the dildo can be inserted before the harness is attached to the leg.
The advantage of this device is that it leaves vagina and anus accessible. When two women make love they can penetrate each other simultaneously.
Some people find it easier to thrust with their leg than with the pelvis, because they can do it with more force and stamina. They can decide how high or low they attach it to their leg to move easily.
The girl penetrated can also do the moving, riding the dildo her lover has attached to his or her thigh.