A siwash harness tightens on the dog like a chinese finger puzzle as the pulling gets heavier. It was developed for sledding with multiple dogs and not for single dog pulling. The term "parade harness" or "buckle harness are derogatory terms used by advocates of siwash to put down a harness that provides a direct pull from the chest.
In 1976 I rejected the siwash for carting and developed a "carting harness" designed specifically for dog carting. It not only provides a direct pull from the chest but also a half saddle over the shoulders so that any down weight from the shafts, especially when a child suddenly jumps out of a sulky style cart, is directed to the correct part of the anatomy as a dog is the only draft animal without a stiff back.