This is a versatile defensive device that combines pepper spray with a rod designed for blocking and striking. Hold the rod in your fist with both ends protruding and your thumb near the button. Use a "hammer" blow with the bottom end for both blocks and strikes. The same grip can be used when activating the pepper spray.
This pepper spray is rated at 2 million SHU (Scoville Heat Units), which is not as strong as the Fox Labs version, but still fairly effective. The ASP Defenders include an excellent locking mechanism that makes accidental discharges very unlikely while still allowing quick activation in an emergency.
The Palm Defender is 115mm (4.5") long and the Key Defender is 140mm (5.5") long. Otherwise they are identical.
This is a tough keychain/pocket-clip holder containing an 11g (1/2 oz) can of Fox Labs pepper spray. This spray contains a 2% OC solution rated at 5.3 million SHU (Scoville Heat Units). It is more than twice as hot as most other sprays. The container is rather thick for comfortable pocket carry.
This is probably the simplest of these devices to use. Grasp it in your hand with the "point" extended between your index and middle fingers and both cross-bars inside your fist. Punch for both blocks and strikes. The "point" is designed to line up with the bones of your arm and wrist for maximum power.
If someone asks about your peculiar keychain you can always tell them it's a Pagan fertility symbol, but security personnel are likely to know what it is. Devices of this sort are now illegal in many places.
When used for blocking or striking all of these items work on the same general principles. Hands are relatively soft and easily damaged. Blocking or striking against anything hard is likely to hurt and may incapacitate you. Using a weapon protects you from most of the impact. You can block or strike against any part of a target without hurting yourself.
The other aspect of these devices is that they focus the area of impact down to a much smaller surface than your hands. When you apply the same amount of force to a smaller area you achieve a far greater effect.
All of these can be used with little or no training. They are designed to be used naturally as extensions of your body. However, training in almost any system of self-defense will dramatically increase their effectiveness, as well as giving you a wider range of options in a defensive situation. Self-defense training also gives you a "weapon" that can be carried anywhere and cannot be taken away from you.
Make sure you are familiar with your local laws before you decide to carry any self-defense weapon. Those listed here are presently legal in most areas of the US, but there are exceptions. Also, there are may be better options available to you that I have not listed because my local laws prohibit them.
For most people under most circumstances the best defensive weapon is a firearm. If you are willing to use lethal force to defend your life you should consider this option. Check into your local regulation and licensing of firearms for home defense and concealed carry. And if you do decide to own a gun get training in how to use it safely and proficiently then practice with it regularly.