This competition is about "tractive effort" rather than speed.
Competitors pull a wieghted sled down a dirt strip until their tractor stops or stalls.
The sled's weight moves forward via a chain drive from the rear wheels of the sled. As the weight moves, it is applying more pressure on the skid at the front of the sled, creating more drag for the tractor. The movement of the weight is measured in inches, which becomes the competitor's "score."
The antique machine's in this competition were 50 or more years old!