From StrategyWiki, the free strategy guide and walkthrough wiki

Prerequisites
Legal entry to San Fierro (i.e., completing The Green Sabre) Location: Stadium in San Fierro (located at the trophy cup on the map)

This is a demolition derby with a computer-controlled team-mate riding shotgun, except they are actually packing SMGs. The object is to collect checkpoints and avoid damage to your car. You start with 0:30 on the timer and each checkpoint earns you 0:15 more. If it gets to zero, you lose. If it gets to 1:00, you win.

There are car repair powerups (glowing wrenches) on the course, too. Pick up these to reduce the damage to your car.

[edit] Tips

You can go about destroying the cars, but remember the goal is to collect time. Avoid the other cars and aim for the checkpoints.

[edit] Rewards

• Cash: $10,000
• Progress towards 100% completion