Oi-zuki (Lunge punch)
Start in a Zenkutsu-dachi (Front-leg-bent stance) with the seiken (proper fist) of the forward leg chambered palm up, covering the ribs on that side of the body. The other arm is extended forward in a ready position with the elbow bent at a 90° angle. The shoulders and hips are twisted slightly with the chambered side back, like a loaded spring. The punch starts by rotating and unwinding the hips to a full forward position which unwinds the shoulders and launches the fist straight forward. The arm should slide straight forward along the ribs, like a piston, keeping the elbow against the body. The fist remains palm up until the elbow clears the body. Just before impact the fist is snapped over palm down.
Gyaku-zuki (Reverse punch)
Gyaku-zuki (Reverse punch) Video
This is similar to the Oi-zuki except that the opposite fist is used.
Start in a Zenkutsu-dachi (Front-leg-bent stance) with the seiken (proper fist) of the back leg chambered palm up, covering the ribs on that side of the body. The other arm is extended forward in a ready position with the elbow bent at a 90° angle. The shoulders and hips are twisted slightly with the chambered side back, like a loaded spring. The punch starts by rotating and unwinding the hips to a full forward position which unwinds the shoulders and launches the fist straight forward. The arm should slide straight forward along the ribs, like a piston, keeping the elbow against the body. The fist remains palm up until the elbow clears the body. Just before impact the fist is snapped over palm down.