Definition: In RC truck pulling or tractor pulling, the sled is the piece of equipment attached to the rear of the truck or tractor. The sled is pulled down the track as far as the RC is able to pull it. A specially-designed weighted trailer, the weight used on the sled is usually a defined amount set by class of truck or tractor being used for the competition.
As the RC truck or tractor pulls the sled down the track, gears work with the rear axles of the sled to move the weight towards the front of the sled (and the rear of the pulling vehicle) to the point that the friction of the front of the sled is too great and overcomes the vehicle pulling it.