If you purchase at $30/bitcoin, and by the time the bank transaction goes through the price has risen to $32/bitcoin, you still get them for $30. Similarly, if

If you purchase at $30/bitcoin, and by the time the bank transaction goes through the price has risen to $32/bitcoin, you still get them for $30. Similarly, if the price drops to $28, you still pay $30/bitcoin. So you're locked in at the exchange rate at the time you place the order.