I think that Turbo Cache works by borrowing additional memory from the system memory to use as video memory.
Basically, the video card will have a default frame buffer size of 64MB, however if video needs become more than the frame buffer size it will start borrowing addition memory from the system ram. It can borrow max 192MB of system ram + 64MB of Video memory making Turbo cache 256MB.
The borrowed Video memory should be returned as system memory when it is no longer needed.