Taekwondo Do san: Step 7 & 8 Backfists of Fury
Movements like this are very practical and way easy to learn as they use 'natural' reiterative motions to achieve economy of motion while allowing the practitioner to throw something out quickly as a 'catch all' technique.
The striking area is either the back of the knuckles IF you are coming down on the nose or soft areas of the face. Otherwise, you can perform the backfist with the front face of the knuckles - the benefit being the student *should* be more comfortable withstanding pressure there. Work both faces to enjoy the versatility of this technique.
Do san Diagram
TDA Training: The Hammerfist from a Traditional Viewpoint
TDA Training: The Practical Backfist
Karate Thoughts Blog: Tapping the Egg