I've read that it helps with anxiety, not barking. Some say it works on the same principal as swaddling an infant, by making them feel confined and safe. Originally it was made for dogs that got anxious in electrical storms, and some say that a tight shirt keeps the body hair from rising, which can be uncomfortable.
I really believe that training helps with barking. Teresa Ford wrote this thread
http://www.yorkietalk.com/forums/gen...h-barking.html and it really helped with Joey. Some things like coins in a can or mechanical bark repellent worked quickly, but then stopped working. The training has taken time, but after a month of consistent training, I saw real results.