The Kettlebell is a system, principally based or as Rif would say a method.
Priniciples of usage
1. Root yourself to the floor with your feet.
Make sure you feel the balls of your feet and your heels in constant contact. This will help your to engage your hips (glutes), help keep the KB travelling a properly aligned path, therefore not pulling you off balance and will help your knees track properly which enables you to contract the appropriate muscles in the body.
2. Link your body together. Make sure to fully contract your muscles which will link your body together and lock your joints helping to distribute the forces throughout your body and not placing stress on a particular joint. Think proper alignment and positioning, which relates to physics (center of mass, center of gravity, flowing motion, leverage, etc.)
3. Tension. Create full body tension. For instance when doing a single arm swing make sure to engage the opposite side, do not relax it. This will get your body linked together and make you much stronger. This will enable to get a much better muscular contraction (i.e. use more of your muscle fibers) which will help to create greater muscular tone/density. So make sure to finish each move with a linked, rooted and tense body. You should feel strong and in control with the stress of the movement distributed throughout the body, not isolated.
4. Breathe. Breathe deep into your belly (think belly button), by sniffing in air through your nose and exhale with some force through your mouth and feel your abs stay tight, stay strong. This breathing will help make you stronger, more forceful and most important will help to protect your core, your spine by volumizing that area with air which creates pressure and protection. Make sure to breathe strong and deep so you can use your core the way it is supposed too.
All these principles work together and when combined with each movement you will become stronger, more skillful, have better technique and get greater results. Your body will become on lean, mean fighting machine :-)
This is a reflection, there is more to this and I am sure that my coach can and will add more :-), but it is a start and these are key to successful practice and progression.
Train smart, and live to fight another day!