A handcuff knot is a knot tied making two adjustable loops in opposing directions, able to be tightened around hands or feet. The knot does not have a locking action, and is not really used to tie people's hands.
The knot is also known as a hobble knot for similar reasons, from the idea that the knot was sometimes used on the legs of horses to limit the distance their riders had to walk in the morning to retrieve them.
The knot consists of two simple loops, overlaid, and with the ends pulled through. At that stage, the knot is slippery and easy to adjust. The knot can be "locked" by making overhand knots with the loose ends to form a reef knot.
The sizes of the two loops can also be fixed by making half hitches with each end over the necks of the loops. This is known as the fireman's chair knot.
Handcuff knot "locked" with an overhand knot
Handcuff knot "locked" with half hitches, this is also known as the Fireman's chair knot