How to convert between two currencies using API?

Hi ShapeShift Team,

First off, seems like you are doing some really cool work. I am new to the world of crypto, so excuse me if this is a basic question, but I'm trying to understand how exactly your transfer API works. Say I have an ETH wallet, and I want to change 1 ether into bitcoin. Does the bitcoin go into the same ETH wallet? If so, can I see the balance? Looking at your documentation, it's not clear to me what the source of the transfer ETH is and what the destination of the resulting bitcoin is:

example data: {"withdrawal":"AAAAAAAAAAAAA", "pair":"btc_ltc", returnAddress:"BBBBBBBBBBB"}

Is the withdrawal address the address of my ETH wallet? If not, where does it know the source of the ETH to change into bitcoin (note that returnAddress is listed as an optional parameter). Please explain how this works.

Happy Game of Thrones day, looking forward to hearing from you :)