Track cycling event; a continuous tag relay for teams of two (or three, on rare occasions on large tracks), generally run over distances of between 20 and 60 km or so (the world championship is held over 50 km). The event forms the core of six-day races, has been featured in the World Championships since the mid 1990s and was introduced into the Olympics in 2000. Named for Madison Square Garden, where it was introduced in the early years of the 20th century after the original single-rider six-day races were outlawed for safety reasons. Depending on the track, riders change with their partners roughly every 500 metres, which means that the race speed can be kept substantially higher than can be maintained by a single rider over any comparable distance. The change is usually effected by a handsling - the slower rider holds his left arm out behind him, and as his team-mate catches him up, he grabs the proffered hand and pulls him up to racing speed in one action, transferring as much momentum as possible. Amateur riders are sometimes required to change by the outgoing rider shoving the incoming rider by the seat of his shorts instead, a slightly safer but less efficient manoevre.
The race is scored as a points race with sprints for points at set intervals throughout ther race and the team with the most points at the end winning (regardless of who crosses the finish line first, although obviously that is in itself worth points). However, in practice the race is primarily determined (especially on smaller tracks) by laps gained and lost on the field, which take precedence over the points score; there is no prohibition on racing directly or taking pace from riders who actually laps ahead or behind.
The actual visual effect of half of the riders on the track racing and the other half pootling round just fast enough to stay upright, with active riders handslinging in their partners as they catch them, with the lined out field behind them sweeping through and around the melee, is a complete nightmare for the uninitiated, but becomes an intriguing balletic spectacle once you have had your neck bearings greased by a reasonable amount of beer and learnt to follow what is going on.
Short-track speed skaters have more recently adopted the same format, but they call it a relay, and the changes are made by a push from behind rather than a handsling.