That is Dynamic Refresh Rate Switching (DRRS) technology, a power saving technology provided by Intel, that lowers the pixel clock frequency and frame rate to a flat panel display in response to power policy, and display activity.
On platforms that support DRRS, the change in refresh rate is accomplished with a mode set change. During the mode set change the graphics chip will stop providing display interface signals. On a LCD the Timing Controller (TCON) in the flat panel display will detect the loss of signals and drive the panel into a safe refresh mode. As a result of the actions of the driver, OS, graphics chip, and TCON, the switch between refresh rates will incur a visual impact; typically the display is black in a few frame periods.