In the popular imagination, the Kentucky Derby is usually won by a horse who makes a powerful late charge through the stretch at Churchill Downs. When the race draws a congested 20-horse field, however, many handicappers look for horses who have enough early speed to stay out of trouble.

In fact, both of these notions are fallacious. But there is a particular type of horse who will win the Derby in almost any year when there is no standout, like Seattle Slew or Secretariat, who simply has more ability than his rivals. The historical evidence is undeniable.

1980: Genuine Risk rushes past the tiring leaders on the turn, opens a clear lead and holds off Rumbo's late rally.

1981: After the speed horses kill themselves with a suicidal early pace, Pleasant Colony rallies from 17th place. Jockey Jorge Velasquez steers him through heavy traffic and the colt accelerates powerfully on the turn to take command.

1982: Gato del Sol comes from last place, circles the field on the turn, takes the lead in the early stretch and pulls away.

1983: Marfa accelerates past Desert Wine and Sunny's Halo on the turn, opens a clear lead and holds on to defeat Play Fellow and Caveat.

Year after year, horses win the Kentucky Derby by coming from far behind and taking command of the race with a sudden move on the turn. Speed horses don't win unless they are truly superior animals; in big fields, they are usually trounced. True stretch-runners don't win often, either. Fourteen of the last 15 Derbies have been won by the horse leading an eighth of a mile from the wire.

There is much logic behind these phenomena, and it suggests that the same type of race will develop Saturday. In a large field, there will always be a number of speed horses, and they have to be hustled early to get a favorable tactical position. The hot early pace will start taking its toll after the horses have gone a mile, as they are entering the final turn. A horse who can accelerate swiftly at this point can seize command of the race.

This scenario should develop again in the 109th running of the Derby. Desert Wine and Total Departure will almost certainly set a fast pace. Both of them are good, tough milers, although 1 1/4 miles is beyond their capabilities. Sunny's Halo will be stalking them, and will probably challenge them on the backstretch.

No 3-year-old in the country looked more impressive in any race than did Sunny's Halo in winning the Arkansas Derby, and he might be good enough to beat this whole group under different circumstances.

But he is going to have to take on the tough front-runners and then try to hold off all the stretch-runners, and that is a formidable task.

The leaders will probably start weakening on the turn, and that is when somebody is apt to make the winning move--a move of the sort that Marfa and Play Fellow made in tandem in last week's Blue Grass Stakes.

Play Fellow won that day, sneaking through on the rail while Marfa tried to lug in badly through the stretch and prevented Velasquez from riding him properly.

That could happen again, but it probably won't--because of the jockeys.

Even in his prime, Jean Cruguet didn't ride very effectively in heavy traffic. Now he is 43, making a comeback after a short-lived retirement, and he gave Play Fellow a couple of disastrous rides before he won the Blue Grass. It is hard to imagine him avoiding trouble as he tries to come from the back of a 20-horse pack.

Marfa is probably a better horse than Play Fellow, having scored decisive victories in the Jim Beam Spiral Stakes and the Santa Anita Derby before the Blue Grass, and Velasquez is certainly a more dependable jockey than Cruguet.

Velasquez is a master at riding in heavy traffic. Even with the No. 18 post position and a horse with some bad habits, he figures to get running room so that Marfa will be able to move on the turn at the right moment. It will be a move that wins the 1983 Kentucky Derby.