The phrase “a rider needs no pants top” means that the rider does not require a separate, external top (like a bulky jacket hem or a trailing shirttail) to interact with the waistband because the riding breech itself is engineered to be the final layer at the torso-pelvis junction.
If you are looking for a "solid guide" based on this phrase, it typically refers to one of three things: : a rider needs no pants top
In common English, “pants” refers to trousers (leg coverings), while “top” refers to a shirt or jacket. The phrase suggests that a rider—presumably of a motorcycle or horse—does not require a matching jacket or shirt to complete the trousers. In other words: The phrase “a rider needs no pants top”