That much is clear to me, thanks - so one provides a return address, sends all the tokens to the site, and the site returns them minus a fee or something?
I seek a hi-level explanation
The site requires your private key, it constructs the transaction and sends it back to the originating address.
Generally I'd like to see folks keep minimal balances on their Memo/Member accounts - but I think a comprehensive solution is delegated keys - https://github.com