Locking was initially called Campbellocking after its inventor, Don Campbell. Campbell was a dancer who appeared on numerous shows in the late 1960s and early 1970s, but performers like James Brown, who also used similar techniques in performing his music, may have inspired him. Locking is often associated with break dancing today, but it occurred much earlier than many of today's forms of hip hop. It’s still used regularly and unlike some versions of hip hop, may be more often performed to funk or pop music than to rap.
The basic locking form is to hold a movement, usually well coordinated with music, after some fast dancing movements. The dancer remains, for a few seconds, locked in place, before resuming dancing. Dancers often work extremely hard to coordinate these moves with music and sudden freezes, that emphasize form, isolation, and music. So it can be said the form of dance is extremely musically aware.
There are a number of lock steps which are often combined with popping. In fact locking is often thought to have inspired popping. When a pop and lock are used, a dancer isolates a muscle movement with a hard hit. Imagine hitting a wall, which stops your movement. Popping essentially provides an invisible wall, so the muscle isolation pops or hits. This movement then may be locked, or frozen for a few seconds, based on inflections in music used.
Some dancers argue strongly that pop and lock, though emerging at approximately the same time in dance history in the 1970s, are really two separate forms. A pop is much more of a serious dance, whereas locking is more fun, celebratory and playful. Plus, most forms of popping consist of series of pops, one after the other. Some describe it as a series of freeze frame photographs. A lock tends to occur through a dance as isolated stops in what are otherwise a variety of fluid and sometimes frenetic moves. Some merge the forms, but others argue that the two forms should exist separately.
There are a huge variety of lock steps, and many of them involve collapsing the body before locking in place in one way or another. The dancer then emerges out of the collapse into a lock. Other features in locking include splits; look at James Brown and Prince performances for these forms of lock steps. Landing on the knees, and holding various other positions, including some taken from mime are also common.