The back fist is one of the most suprising punches you can deliver because it is fast, accurate and nontelegraphic. The blow can be directed anywhere on your opponent's face but the temple is the best target. (Bruce & Uyehara 1991 vol 4)
Due to the nature of delivery the back fist is not the strongest punch that can be delivered therefore to increase its effectivenes it is often used in combination with lap-sao (grabbing hands). This way you can jerk your opponent forward into the punch increasing its effectiveness.
