This is a climbing device for belaying with.
The nice thing about the gri gri is that it will
automatically lock the rope if the leader falls.
It works by having the rope wound around a shunt like
coil which is free to move, if the rope moves through
quickly (as in a fall), the coil is raised and the rope
is squeezed between the coil and the wall of the gri gri causing the rope to break.
It is hard to feed the rope backward through the gri gri.
The gri gri is made by Petzel