Usually a handicap is a calculation that some math whiz comes up with the to create and even playing field.
In the last handicap race I saw, they gave the woman a 9 minute handicap. Which meant, they started 9 minutes ahead of everyone else, so that supposedly everyone would end up at the finish in a neck and neck race.
Seeded races are common in triathlon and are actually the OPPOSITE of using a handicap. As a matter of fact, the slower you are the later you start. If you are "seeded" with a lower number, you start earlier because you are quicker. This strings things out on the race path so that no one is tripping over anyone else.
In a handicapped race, the slower ones or the ones given a larger handicap will start first in hopes that the quicker ones who start after you will catch up.
This way, they can stop awarding segregated awards like age groupings and male/female awards. They just award the overall winner, which is whoever crosses the finish line first. I have seen many seeded races where the first person who crossed the finish line wasn't necessarily the winner.