A person (male or female) who endeavours to move credit card balances around from one card to another (so-called Balance Transfers) calculating the cost to transfer so as to minimse interest payments. Rate Tarts will 'drop their knickers' to do this means that they will move the balance continually from one card to another, and have no loyality to any one card or company, ie they'll drop their knickers for anyone!
Eg: a rate tart will look and see that Credit Card Company ABC offers a 0% Interest rate for 18 months, with a 2.99% fee to transfer, as opposed to Credit Card Company XYZ who offer 12 months interest free and a 3.00% transfer fee, they will choose ABC.