AMD uses a new form of power management on their RV6xx ASICs called DPM. The GPU independently senses the GPU load and will pick from a set of predefined clocks according to load. Also clock gating is used which turns off unused parts of the silicon and quickly turns them on when they are needed. Unlike the 2D/3D switching on previous ATI cards where the driver manually switched clocks as soon as a 3D fullscreen application is started this process is completely transparent to the software and the driver.
This also means that windowed 3D applications will run at the intended performance.