This involves chemicals being rubbed into the sensitive tissues of the coronet band - chemicals such as mustard oil, croton oil, salicylic acid, diesel oil, etc. These agents burn and blister the horse's skin, causing intense pain to the horse, who therefore raises its fronts for the big front end action sought for in the show ring. Furthermore, these chemicals are often carcinogenic and very toxic.
One side-effect of using chemicals is scarring. Therefore a horses' lower legs may be soaked in an acidic bath and left in its stall - likely unable to stand and in excrutiating pain - to recover as its skin desintegrates. The hair later regrows, and in this fashion any soring scars are left invisible.
Another side-effect of soring "stewarding". As horses may be palpated prior to entering a class; a sore horse is likely to flinch when pressure is applied to sored areas. So, prior to a show, a trainer may palpate the horse himself, beating the horse any time he flinches, thus teaching the horse to forego flinching when palpated prior to entering the ring. Another way of avoiding detection is to use a temporary freezing agent over the sored areas.
Soring the Gaited Horse This is a highly informative site that includes the history of soring
Soring TWH's Wikipedia
For the Tennessee Walking Horse A blog with information, articles, and up-to-date info on the practices of soring
Natural Horse Talk Source-backed info on gaited practices
One final note: these practices are certainly ot restricted to the gaited world such as Tennessee Walkers and Saddlebreds. I have personally seen Arabians undergo some of the above methods and I understand Morgans are not exempt either.