As the voltage is decreased to 4.9 V, only some of the least significant bits are cleared. The MSB will remain set until the input is one half the reference voltage, 2.5 V.
This describes how a system responds to a change in voltage, specifically that the least significant bits are cleared when the voltage drops below 4.9V, while the most significant bit (MSB) stays set until the input reaches half the reference voltage of 2.5V.