"PayPal Instant Transfer is a feature that allows you to use your bank account, but send the money right away. It only works if you have both a bank account and credit card registered with PayPal. Here's how it works:
You choose Instant Transfer. The money is sent right away, and PayPal starts a bank transfer. If for some reason the bank transfer fails (you don't have enough money), then it will use the credit card. So it is like a bank transfer that is backed up by the credit card.
Note that PayPal deducts a fee for this service, but it is the person receiving the money that pays it (they get a little less than you actually sent)."
(you need an account to see links)
For tl;dr, you need to add a credit card AND a bank account and it should be an option to instantly transfer funds from your bank when your paypal does not have enough funds. From what i see it is a specific option that you have to choose when making a payment. Hope this helps!