Backfist is performed by extending the cocked arm (generally starting from in front of body, palm to solar plexus) outward to side to extension where wrist snaps as the arm is pulled back toward you, striking with the first two knuckles on the finger side (same point of contact when punching). It can be performed at various angles, including horizontally (from solar plexus) and vertically to front or side (starting fist to shoulder). This punch is also applied with a whipping motion. Application note: Backfist in goju is usually applied like a jab, with little power. To generate greater power, apply body movement (either in rotation or forward) until contact is made. Spinning backfists (where body spins around and hand blindly strikes), the backfist is fully extended and continues through point of impact to return in downard arc to chamber.
We typically practice this strike to the side. Both elbows extended to side, held high, fists to shoulder. Strike outward to side in a snapping motion, then bring quickly back.
Then practice from an elbow foward position, striking to the front, as shown here.