Hi you send your btc to a multi sig address that's distributed over 3 or so servers. Then after 6 confirmations you get mgwBTC asset and you sell/bid/ask that for nxt on the asset exchange. Trades take as long as nxt blocks. Look into instantDEX for real time decentralised trading. 
So the trades at mgwBTC are not instantaneous? Also, how the distribution of bitcoins between 3 (or more) servers helps prevent robbery? It may make it more difficult to steal all the bitcoins from all the servers, but this still doesn't prevent server hacking AND stealing the funds, if I'm not missing something. My point being that I lose control over my funds once they are sent to the exchange. Is this not what decentralised exchanges are supposed to get rid of?