You can spend sats from the USD account or send USD to other Kollider Pay accounts. The thing that changes between BTC and USD accounts is the unit of account -- so if you deposit to the USD account 5000 sats and that's the exchange rate for $1, your USD account stays at $1. if bitcoin halves in price, you have 10000 sats in the USD account spendable. Goes the other way of course, but that's why you have multiple accounts. You choose your allocation of holdings, and if you want protection on a potential price drop you can secure it.