"grinding their pelvises together" HAHAHAHAHA
Jacking is a dance technique that was popular in the late 1980s. "Jacking" came from the early Chicago house music scene. The dance itself was sexually charged, with couples (often two men or two women) grinding their pelvises together. The style gave birth to the dance styles popular during the early UK acid house scene.
When referring to house dancing this word has taken on a slightly more specific and less sexual meaning. Jacking is the technique that comes from moving the torso forward and backward in a rippling motion, as if a wave were passing through it. When this movement is repeated and sped up to match the beat of a song it is called jacking, or "the jack." This technique is the most important movement in house dancing because it is the foundation that initiates more complex movements and footwork.