The way I learned it from Minowa sensei, you're essentially delivering a vertical fist strike with the Tikko acting as a force multiplier. The only caveat is that the strike is vertical and doesn't use the two primary knuckles for impacting. Because you're using the Tikko, the fist impacts flat onto the target (a big "no no" in Karatedo) . The other point is that the thumb rests on top of the handle of the Tikko to stabilize it.
Tokushin No Tichu – Tokumura Kensho