The event is sometimes called the 1000-metre sprint (reflecting the distance to be covered), but only the last 200m is actually timed (on a 250m track, the event would therefore involve four circuits of the track). The early parts of each race will often be highly tactical with riders pedalling slowly, some riders even bringing their bicycles to a complete stop (a difficult skill, as their feet are strapped or clipped to the pedals of their cycles). The reason for this strange behaviour, like many track cycling events, is aerodynamics.
When racing at high speed, the rider who manages to stay just behind their opponent can 'slipstream', expending less effort (as they are riding through less dense air because the 'lead out' rider has pushed it out of the way). Just before the finish, the trailing rider pulls out of the slipstream, and using their fresher legs may be able to overtake their opponent just before the line. (Of course, some riders choose to accelerate quickly before the last lap, hoping to catch their opponents unawares and establish a large enough lead to negate the aerodynamic effect!)
The final 200m is measured along a 'sprint line' close to the inside edge of the track. The 'lead out' rider will often hug this line forcing his opponent to come the long way round; he is not allowed to swing significantly off this line and deliberately impede his opponent.