Massage is a power used to enhance circulation and strength in the body. It is initially used by Tsukino (via electricution) to help Kyo improve his muscle mass while training to attain the Solar Gauntlets.
Tsukino's style uses a device similar to the Dual Muscle Therapy marketed by Dr. Ho, a Toronto chiropractor.
Later on in the anime (though it is a flashback to a time that occurred in the past) Kid uses it to help heal Bolneze after he tore every muscle in his body during harsh training where he tripled his cloning ability from 50 to 150.
Bolneze at first assumes that Kid is using a type of French massage, but he is actually using Thai massage.