Consegui encontrar um tópico nos Fóruns do Windows Vista onde exatamente a mesma pergunta foi postada.
A resposta dada é:
GPU Bank Flip; Also known as Graphics Processing Unit, is designed for Crossfire Technology from ATI, certain Circuit Boards which support this is set as Disabled as default for user select for crossfire in later cards, which allow the two ati cards to switch from one unit to the next to use and combine both 1 gb or 512 mb of ram on the cards itself.
Parece que permite que a memória de uma placa gráfica seja usada por um segundo.