The frame buffering is a physical feature of the CCD. It actually has twice the number of lines, but half of them are behind a mask so they will never be expose

The frame buffering is a physical feature of the CCD. It actually has twice the number of lines, but half of them are behind a mask so they will never be exposed. When the exposure ends, it rapidly shifts the accumulated electrons from the exposed lines into the area with unexposed lines. From there, they can be read out by the ADC while another exposure is in progress.