Some Quick Thoughts on Some Body Weapons.

The palm-heel is probably one of the most useful body weapons that you have. It is a technique that nearly anyone can use, can apply great power and it useful for both hard and soft targets. Suppose you encounter an aggressor wearing a motorcycle helmet and an opening occurs for you to hit his head? If you use a punch there is a good chance you will damage your hand. Most people, however would be confident at striking the helmet with a palm heel and can probably hit with enough force to jar his neck or knock the attacker off balance. If you can do that, think how effective a palm-heel strike would be against his unarmoured jaw, kidney or top of the sternum.
Palm-heel is one of your primary weapons and should be used whenever possible and practiced until its use is second nature.

Hammer-fist is a somewhat neglected weapon in many martial arts but is one to master. Hammer-fist can be used in many of the applications usually suggested for a back-fist or knife-hand strike. It is more powerful and more versatile than a back-fist and easier to use effectively than a knife-hand. Its larger impact area means that it is often recommended for applications where permanent injury is undesirable. Many Police officers are taught to use the hammer fist or palm-heel instead of the knife-hand when striking the brachial plexus or vagus nerve, for example.

The main applications of the knife-hand are against the limbs, neck, kidneys and between the ribs. In many manuals the knife hand is shown with the fingers apparently straight and the thumb sticking out. I suggest you experiment with the palm slightly cupped and the thumb tip placed just behind the middle joint of the first finger.
Practice knife-hand by striking it against the palm of your other hand. This also conditions that hand for palm-heel striking.

Back-fist is a staple of many martial arts but is in fact a somewhat limited technique. Hammer-fist is an easier, more versatile and more powerful technique and generally more useful. To use back-fist effectively requires “Loose wrist, Tight fist”. The hand needs to be clenched tightly on impact but the wrist needs to be relaxed to produce and in and out snapping action. Striking area is the first two knuckles and if you are relaxed enough the front rather than the back of the knuckles will hit.

One-Knuckle Fist.
Because it is a relatively low powered technique for most of use it is not good against large volume soft targets like the torso. Main soft targets are the nose and the larynx. Most targets for the back-fist are boney areas such as the mind-point of the jaw, eyebrow temple, sternum, philtrum etc. Since it applies a relatively small bone against larger bones good technique is essential. In most self-defence techniques where you see a back-fist used see if a hammer-fist can be used instead
One-knuckle striking techniques such as Phoenix eye fist or middle-knuckle fist are seen in many martial arts. These techniques are best used with a circular rather than linear punching technique, particularly against harder target areas. Bend your wrist slightly so that the forearm, hand and promimal phalanx form a smooth curve.