As a secondary wallet, if a user wants to spend Bitcoin, their "large" wallet can sign a transaction with the user's private key and put it on the Bitshares network.  The Bitshares witnesses can create and all start signing a multi-sig transaction to spend the Bitcoin on the Bitcoin network.  This takes the balance away from a server that could be hacked and instead puts it into the hands of many witnesses (which would all need to be hacked).

I'm not sure how this would all work under the hood.  More research is needed on this...