A gyaku zuki or gyaku tsuki is a karate

Karate

is a martial art developed in the Ryukyu Islands in what is now Okinawa, Japan. It was developed from indigenous fighting methods called and Chinese kenpō. Karate is a striking art using punching, kicking, knee and elbow strikes, and open-handed techniques such as knife-hands. Grappling, locks,...

technique. It involves a punch executed by the back arm. A gyaku zuki is sometimes called a "reverse punch". It is also executed better if you use the hips to push it forward.
It is one of the most used and effective punches used in a karate fight. The punch is generally to strike the solar plexus. The movement of a Gyaku Zuki consists of muscles such as the gluteus maximus, calf muscles and the thigh muscles. For long range punches twisting is important.