I think the position that Cathe uses stresses the long head of the tricep more, and the other position stresses the short head, though I wouldn't know where they are one in relation to the other ;-) An alternative I like is to start in Cathe's position, then put a twist in so I end up in the Firm's position. That way, you are working two of the movements the triceps are responsible for: straightening the arm, and rotating the arm (is this pronation or supination? I think it's pronation!). The third movement the triceps is responsible for is lifting the straight arm back behind you, so if you wanted a really all-inclusive triceps kickback, you could put a little pause in after you get to the top (of the twisted position), then lift the straight arm up and down for a pulse).