Derri Sandberg demonstrating the KB Swing
I see many people doing what they think is a KB Swing, but is actually completely wrong and sometimes dangerous looking. The biggest mistake I see most people make is squat and lift the KB with their arms. The Swing is NOT a squat, but rather a hip hinge. The weight is propelled upward by the hips rather than by the arms.
Imagine a rod running through your hips from the side and if you squatted it would cause excruciating pain. However, if you just pivot on the rod everything feels fine. This is what I have my clients imagine when doing a Swing.
Learn proper mechanics of the KB Swing and watch your strength and cardiovascular fitness improve dramatically. Your hamstrings will become very strong, as will your grip strength.
If you want to learn how to do it properly, seek the advice of a trained fitness professional or anyone with an RKC certification.