Well, with a Credit card being linked, once your balance hits $0 on PayPal, it will start to use your card. Similarly, if you have $10 and you're buying something thats $15, it'll use your balance and then $5 on your card.
This doesn't work if the purchase is in a foreign currency (that is, one that is NOT your paypal primary balance currency), it then has to either be fully on balance, or fully on card.
If you ever sell something here, keep in mind that people CAN call their card companies and have charges reversed if they paid through PayPal and a linked Credit Card. This is why many big sellers here want it to be a balance-funded PayPal.
Hope I helped =]