These are one of my favourite triceps exercises as they force the triceps to do a lot more work than the equivalent dumbbell or barbell versions you see many people do.
At the top of the movement, a dumbbell or barbell would "sit" directly down onto the arms so the triceps would get a momentary rest.
Due to the position of the kettlebells, this means the weight is constantly engaging the triceps muscles to stop the arm bending.
Therefore working the muscles harder.
This is my, mostly, Personal Trainer musings and information which I hope you'll find helpful!