Paypal - switching between live and sandbox in MarketPress

I've got the PayPal API signature and passwords for both Live and Sandbox use. Both work fine when entered into the Marketpress Payment settings, but I don't understand what the 'Mode' setting is for. What I'm expecting is to enter BOTH sets of keys, for live and sandbox, in different fields, and then to use the mode radio buttons to switch between them. What I have to do at the moment is replace they keys as I change between live/sandbox. Is this right? What effect, if any, is the 'Mode' setting having?