In volleyball, six pack is slang for when a defender gets smashed in the face with a spiked ball. The term was originally used in beach volleyball during the 1960s and today the term has spread into indoor arenas and all levels of competition everywhere. Apparently back then it was proper volleyball etiquette for the spiker to buy the defender a six pack of beer to relieve the pain of a rearranged face. Some people today have taken the reverse stance and think the defender should buy the spiker the beer as some sort of reward.
Six packs can occur either when a defender is trying to dig a ball in the backcourt or when the defender is trying to block the spike at the net. They generally happen either because a) the defender was not paying attention or b) the defender assumed the spike was going to end up anywhere but their face and couldn't recover in time. The best way to protect against a six pack is to watch the direction the volleyball is travelling and the arm swing of the spiker to get an estimate of where the ball will go. This gets harder the faster the ball is spiked and even professional volleyball players get whacked in the face occasionally.
Receiving a six pack generally bruises the ego more than the face. Gentler spikes will merely result in a stinging face and much laughter while high velocity spikes will give the recipient a pretty red face and perhaps a bloody nose. Sometimes the force of the strike is enough to knock the defender to the ground, in this case the incident is referred to as a "twelve pack" (more pain requires more beer!).
Six packs are also called a facial or a Mikasa tattoo (Mikasa is a volleyball manufacturer).
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Personal six pack experience