If you're ever in the unfortunate position of needing to restrain someone, the "handcuff knot" does exactly what it sounds like. This versatile knot can do a few other helpful things too.
This instructional video, from the ITS Tactical YouTube channel, demonstrates how to make an effective restraining knot in a matter of seconds. It's difficult to describe in words, but the idea is to make two loops in your rope, then cross them and pull the loops through each other. What you have left is two adjustable loops you can use as makeshift handcuffs. This knot can also be used, however, for carrying or dragging game when hunting, and as foot holds when pulling someone up from somewhere or lowering them down. Tying basic knots requires very little expertise, so it's worth taking a few minutes to learn the ropes.
How to Tie a Handcuff Knot [YouTube]