The player with the highest number of points after the last round is the tournament winner.
Some set of tie-breaking rules must be chosen in advance to be applied if two or more players achieve the same number of points.
The advantage of the McMahon system over the Swiss system is that it requires fewer rounds to find a winner, and that it avoids extreme match-ups (very strong players against very weak players) in the earlier rounds.
By matching up possible tournament winners earlier, the system allows for more games amongst this group, and thus improves sampling.
In other words, the sorting effect of wins and losses propelling players up and down is applied all along to refine what is believed to be a pretty good initial ordering, rather than being wasted merely to bring similarly skilled players together into roughly sorted order, and thus the McMahon system achieves a more accurate final sorted order.