I left a link in the post about it ... I know what is explained there, not more. https://blog.decred.org/2017/09/20/On-Chain-Atomic-Swaps/
As far as I know you must pay 2 fees, one for each coin, when you do the swap and is not intended to be a replace of the market and the exchange rato you talk with the other part and get in an agreement.

But here there is more technical explanation https://github.com/decred/atomicswap/