A number of complex parameters control when a core should be parked, and Microsoft tuned heavily towards power savings. Their interest was in conserving energy, even if this meant marginally decreasing performance. This technology is very similar to frequency scaling, in that it seeks to throttle the CPU when idle.
Disabled cores are re-enabled as the CPU load increases once again.
Core Parking dynamically disables CPU cores in an effort to conserve power when idle. Core Parking is a sleep state (C6) supported by most newer x86 processors, and newer editions of Windows.
