Dirt Track is a race involving the Sanchez that is one of two events that is available at Blackfield Stadium in Blackfield, Las Venturas. This event is only available on Mondays and Wednesdays and requires at least a 50% Bike Skill. There are six laps and 11 other contestants, with the player having to finish first in order to get all rewards. The rewards are $25,000 for each time you complete it, as well as BF Injection that spawns outside of the stadium upon first completion.
As for the race itself, it's not too hard, as the competitors will likely fall off their bikes when they land the second jump, giving you the chance to take a lead over them. Another strategy is to simply kill the other competitors, although they tend to have a lot more health than normal.