Fold your rope in half, take the U-shaped end (called the ‘bight’) and wrap it twice around your partner’s wrist or ankle leaving room to slip two fingers underneath.
After the second wrap, cross the bight over the top of the wraps then pull it back through the underside.
Leave enough of the bight to tie a square knot with the remaining rope.
The final result looks like this.
The remaining rope and bight can now be used to secure your partner’s wrist or ankle to something such as a bedpost or chair leg.