Complex sentence: When a horse, mule, or other animal is in harness, harness breeching helps the animal to slow or control the forward movement of a vehicle that has either a pair of shafts or a single pole projecting forwards for about the length of one animal.
Split into simpler sentences: When a horse, mule, or other animal is in harness, harness breeching helps the animal to slow or control the forward movement of a vehicle. The front of such a vehicle has either a pair of shafts or a single pole projecting forwards for about the length of one animal.