There is a lot of info on the web about them. I don't think they always help with back drafts. There are some specific models that are supposed to be better. Mine is not a standard top so buying off the shelf wouldn't work so a chimmney sweeper is the best bet. One of the things I remember when researching was keeping heavy rains out which can damage the masonry.
Just another reason to strap on the korkers. Got mine at HD for about $25 and it does reduce the cabin smell in the humid weather. Just don't let your kids see you locking Santa out.
Caps help keep the rain out which extends chumney flu life, no animals, birds, and also keeps down the smell caused by rain soaking into soot and creosote. I burn wood, 3 to 5 cords on average and since I put the cap on thier has been a noticable difference with the draft and smell.
Swimmer a.k.a. YO YO MA
Serial Mailbox Killer/Seal Fisherman