From what I've read Mane and Tail isn't that effective, I used another horse product called Cowboy Magic Detangler when Joey was a puppy, and I thought it worked great. I would use it under his arms, especially after wearing clothing, and before the bath, I used it all over, it's best to get all mats out before bathing. A small amount of detangler used between your thumb and forefinger on the mat going in a circular motion seems to dissolve the mat. Some people use human conditioner or the mat, and some people swear by regular cornstarch. I can't emphasize enough that this should be done before bathing, once wet, mats become much more difficult to remove. There are matting combs that have a blade that cuts through the mat, and these should be used with caution. The easiest way to avoid mats is to brush daily, following with a comb that is placed next to the skin to insure all tangles are out. I always followed shampooing with a good conditioner and put a dab, on those underarm areas.
Joey outgrew the matting, but I still use a conditioner after every shampoo, and of course brush daily. Mats that are close to the skin often have to be shaved, but this is probably best left to professionals, and those who know what they are doing.