It's usually called a global shutter, and it adds complexity to the sensor design, because each pixel has a tiny amount of "memory" with a gate and capacitor to

It's usually called a global shutter, and it adds complexity to the sensor design, because each pixel has a tiny amount of "memory" with a gate and capacitor to store charge into. There is a signal bus that simultaneously moves all the charges from the constantly exposing pixels into the storage areas, where the charge is no longer being increased by exposure, and can be read out asynchronously using a switching matrix and a bank of ADCs. This avoids the problem where the charges of the pixels you haven't read are still increasing through light exposure.