With all due respect--the majority of the show ring harness horses are NOT working back to front--a horse that is engaging his hindquarters properly has a rounded back, which allows his hind legs to get under him, and yes, that does elevate the front end. However--show ring driving horses are not trained that way. Look at them carefully. Hind legs are frequently trailing, not getting up and under the horse. Front end is pulled up by the check rein and lines. Horse's back is hollowed--with a hollow back the horse cannot engage his hindquarters. These horses do not get the dressage training it takes to get the engagement and elevation that you see in a truly light, collected horse--they are simply made to go in a certain frame.
If the traces are not taut then the horse is pulling with something else--either the saddle or the lines. In a level arena with firm footing pulling with the saddle isn't a huge issue--the horses get by it seems. If the horse is pulling from his mouth--that would be very wrong. If the horse is pulling with his mouth then he is not light in the bridle, he is not collected, he is not anything except having his face pulled off!! Please.