McMahon is the name given to a system of running tournaments. It is explained in the McMahon Pairing article in Sensei's Library. Similar to the Swiss system, but starting with points allotted to players in order of presumed strength, almost as if a few rounds had already been played.
If the top group start at zero and the lower groups have negative points, the final points for the top group members will equal their numbers of wins.
A variant is the Aguilar-McMahon system, suggested by Fernando Aguilar of Argentina for a 2005 tournament.