The knife hand is an open hand strike that uses the thick, outside edge of the palm. The fingers are kept tightly together and the thumb is curled and kept out of harm. The knife hand is common to many stand up martial arts styles including kung fu, karate and tae kwon do.
This version of the knife hand is performed with the palm facing upwards. The arm is kept bent, with elbows close to the body. This is typically a close to middle range technique. Extending too far is ackward, much weaker and can cause injury to the shoulder or elbow.