Last year the airlines bumped over 800,000 people. What exactly is bumping, why does it happen, and how can it be a terrific windfall for you? If you can get yourself bumped from an overbooked flight, you are entitled to receive a free flight voucher as compensation. In other words, this is the one case in which you can fly for free, compliments of the airlines, without having to pay them anything. Bumping is the airlines' alternative to compensating you in cash for taking you off a flight for which you have a confirmed reservation.

Bump tickets can be worth far more than the legally specified dollar compensation for overbooking. By offering these tickets, the airlines have nearly eliminated passenger complaints for overbooking. With the reward of a free ticket, enough people are usually willing to relinquish their confirmed space to accommodate other confirmed ticket holders who don't want to wait for another flight. You are put on the next available flight to your original destination and have a new ticket good for another trip at another time.

The chances of getting bumped on any given flight in the United States are slightly less than 1 out of 500, but if you are interested in obtaining a bump ticket, try airlines that allow travelers on crowded flights to volunteer for bumping-usually on a first-come, first-served basis-at the departure gate.

You can improve your odds for getting bumped by following a few guidelines:

• Choose the airline with the worst bumping record at the most congested airport. You can check the bumping record through the Department of Transportation's monthly Air Traveler Consumer Report. The most congested airport would be in a hub city through which an airline connects most of its longer city to city flights, thus eliminating non-stop flights to cities served by that airline. Because of the sheer volume of passengers passing through the larger hub airports, there is usually a great deal more congestion as well as flight delays occurring in the part of the airport used by the hub airline.
• Fly at the most popular, busiest times of the day for the route you are taking. This means, typically, Monday morning, Thursday and Friday afternoon/evening, or the beginning or end of a major holiday weekend.
• Bring only carry-on luggage with you.
• Arrive and check in early at the airport since some airlines require you to have checked in on time in order to qualify for a bump ticket.
• Once at the departure gate, immediately volunteer to be bumped in case of overbooking.
• You are more likely to get bumped during the summer at an airport with extreme heat or high altitude. Hot weather causes thinner air. With thinner air, airplanes need to fly lighter. That means less luggage and sometimes less passengers.

There are, of course, no guarantees that will assure you of a bump ticket. You need to weigh the inconvenience of rescheduling a trip with the reward of a free ticket. The airlines want to issue as few bump tickets as possible, so they try to estimate exactly how many no-shows will figure in their booking process.

When is a bump ticket most useful? When you have to travel on short notice and have only the choice of buying a full-fare ticket rather than an excursion one. With a bump ticket in hand, you are traveling at a much lower cost than full-fare coach purchased at the last minute.

Also, most bump tickets don't require a minimum advance purchase or a Saturday-night stay. In fact, a few only allow 24 or 48 hour advance reservations (which, obviously, wouldn't work in planning a trip very far in advance).