- You have a satoshi (0,01µBTC) that is backed by some issuer with 1USD. This satoshi is stored in an address A which you control.
- I offer you 3BTC for it, and you accept. These 3BTCs are stored in an address B which I control.
- You provide me a deposit address D for the 3BTC, and also inform me of which address contain the backed satoshi (A).
- I create a transaction which has both A and B as inputs, C as output for the satoshi and D as output for the 3BTC. I sign it with B. It's not yet a valid transaction, as it needs to be signed with A, which is in your control.
- I transfer you the invalid transaction, you sign it with A, making it valid, and send it to the network.
OBS: I can't pay only 0,01µBTC for your backed coin, because then the issuer can't keep track of his backed coins anymore. But such trade wouldn't make economical sense anyway.