Doh-san step 7 and 8 are backfists executed in a forebalance. We did this as a drill last Tuesday, varying it and getting the back hand to provide a block/check. So an opponent will throw a centreline punch and the drill is to block the oncoming punch downwards and throw a backfist. This may get deflected or blocked, and the student reiterate the movement with the other hand - the attacking backfist becoming a downward press and the back hand throwing another backfist. This is done tight on the centreline similar to wing chun techniques.
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