Abernethy interprets the spearhands as rotated strikes with the palm heel, with the fingers having no involvement at alland the strikes are not strikes to the midbody, but to the attacker's forcibly lowered head as a result of what he analyzes as controlling techniques immediately preceding. The real striking surface, given the bunkai he presents in his DVD on the Pinan kata set, is the base of the palm.
Seriously, I've always been taught that it was more of a palm heel strike than a 'finger' strike. We do a strike kata where the spear hand is used across the eyes/top of face from the side with the palm doing the striking. In the same kata the spearhand is done palm up to the groin.