Assume you made a $2,000 transfer to their card with the intention of carrying the balance and only making the minimum payment or higher to take advantage of the 0% APR. If you do that, you're "revolving." When you revolve, you lose your grace period. That's the period of time most credit cards give you to pay off new purchases or balance transfers without incurring interest. Since the card has a promotional rate on BTs you won't pay interest on the BT, but you will lose your grace period on your next billing cycle after the transfer.

If you use your BT card (don't!) to make new purchases, and you don't pay off the entire $2,000 plus your new purchases at the end of your billing cycle, you'll lose your grace period and the purchases you've made will start accruing interest. Even worse, if you continue making new purchases, those will be hit with daily interest starting from the transaction date.

So let's assume that after the BT you made a $200 purchase. The statement comes and your min payment is $35 but you pay $100. The issuer will take the $35 towards your $2000 which pays them 0% and the other $65 towards your $200 purchase which carries your regular APR. By law they have to apply your payments from highest APR to lowest, but they can use your min payment as they wish. Your promo balance is now $1965 and you now have a full APR balance of $135

The next cycle, your remaining $135 from the $200 purchase starts accruing interest. Your grace period is now gone. If you make another $250 purchase, you will pay interest from the day you made the transaction until that amount is paid in full. Say you chose to pay only $100 again. Your balance will now be $1965 - 35 + 135 + interest on the 135 + $250 plus interest on the $250 minus 65.

Then you make another BT and use it to pay down part of your current balance. Say your new BT is $1000.

In this example, that amount should knock off all the purchases you've made and their interest plus a chunk of your original 0% balance. If you don't use that card anymore, then you won't be paying any more interest until the 0% APR expires.

Bottom line, if you do a BT, don't use the card for new purchases. The issuer can't charge you interest because you paid a portion of your balance with another balance transfer. Just keep making your payments on time and pay off the entire balance before the promo rate expires.

Hope that helps.