A good yawara stick is hard to find
This is a hard nylon version of the yawara stick. There used to be some impressive versions of these on the market, but now there's this one and two others, one aluminum and one wooden, both functional but rather less inspired. The more common pocket stick is the kubotan, which is just a shorter, skinnier yawara stick, usually configured as a keyring. Kubotans come in many shapes and materials.

As Don Rearic* writes, you can think of a pocket stick as a knife, but with two differences: You can thrust, but not cut; and you can target bony areas. Also, to the extent that poking holes in people is bad legal hygiene, the stick is safer than the knife. You can strike with the "pommel" of the yawara in a hammer blow or with the "point" in a reverse hammer blow or even a sort of rapier thrust, bracing the tip with the thumb. You can also use it to trap or lock joints, if you know what you're doing.

Anything you hit even moderately hard is going to hurt. This thing can break bone, pulverize muscle, and shock nerves, all without skinning or fracturing your knuckles. Yes, the product description says it's non-lethal, but if you whack someone in the head with it, you could kill him. And "just" breaking someone's bones is no joke, either. Before you try it out on friend or foe, do everyone a favor and take a few whacks at a piece of 3/4" plywood. This should give you some respect for what this tool can do.

Unlike many weapons, the yawara stick won't extend your striking range. You have to get up close and personal with whatever you're hitting, which usually means it can hit you. If you're too timid to close range, a pocket stick probably won't do you a lot of good.

This thing is like having big, indestructible knuckles on each end of your hand. Highly recommended.

*Please use Google for more info, as Amazon doesn't allow offsite hyperlinking.