The blinding speed of riken uchi, the backfist strike, makes it the ideal atemi. Like the front-foot punch, backfists are thrown with the lead hand, and not from the hip. This makes backfist one of the few hand techniques available when fighting from a full-side-facing position, Backfists target the opponent’s nose, temples, or the side of the jaw. The backfist's striking point is the riken, the back of the first two knuckles. (Thus, the name.)
To throw a backfist, extend your arm, much like a front-foot punch, but with a loose hand and a limp wrist. Pretend that your hand is a jellyfish swimming through the air to the target. At the instant before your hand contacts the target, clench your hand into a fist, and pull your arm back to its starting position twice as fast as it went out. Your limp wrist will snap the hardest part of your hand at the target like a bullwhip, or a wet towel.
[videos of backfists, fast and slow from the front and from the side.]
[video explaining the jellyfish-like motion.]