There are two options. On BitShares, there are for example OPEN.BTC. This means you send you BTC to some more or less trusted party (called OPEN in this case) that stores them and gives you an OPEN.BTC token. Essentially this is like a gold backed dollar. Your OPEN.BTC represents the right to access your BTC if requested. I am not sure these services can be trusted 100% but certainly better than traditional exchange.

BitBTC are fundamentally different. They are a smartcoin. In order to create one you dont need to have a real BTC in the first place. Instead you lock some Bitshares in a smart contract that secures the value of the BitBTC to the BTC value. In case that the price of BTC goes up more than the backed amount and there is no user wanting to issue new BitBTC, then you get paid out the full collateral.
This will be enough to at that moment buy you a real BTC.

These smart coins are 100% decentralised and safe. In my opinion stay away from services such as OPEN.BTC and trade BitBTC instead. I know the underlying math may seem confusing but I examined all the cases and have not found a flaw unless bitshares value crashes abruptly against all other cryptos.

Note that currently one BitBTC is worth more than a BTC! The value is only backed to the lower side (comparable to SBD). It reflects that a BitBTC is actually better than a BTC as it can be traded and send fast and secure. Both features currently missing in BTC.