This constant-tension exercise is one of the most difficult triceps mass builders there is. Simply lie on either a bench or the floor and do skull crushers in an alternate fashion with kettlebells.
However, there's a catch. In between reps, the arm that's not moving is held at the bottom of the skull crusher, just above head height.
This position, along with the constant pulling tension created from the kettlebells, creates enormous levels of mechanical tension, metabolic stress, and muscle damage, making this one of the greatest mass-builders you'll ever use for triceps.
If you're looking to notch things up another level, try doing them with the lower body elevated in an isometric leg raise position throughout. This increased tension throughout the lower body and core produces a neurophysiological phenomenon known as concurrent activation potentiation (CAP).
That means greater neural drive to the rest of the extremities including increased motor unit recruitment and innervation throughout the triceps. Do just 4-6 reps per arm for strength and size gains.