How is the electronic shutter on a CCD implemented?

The electronic shutter is actually implemented by a mechanism that "dumps" the collected charge at each CCD pixel on each line. This is done by raising the voltage on the substrate high enough that all electrons leave the pixel. The shutter integration starts when you stop dumping or removing the charge. At the end of the integration period, all charges are simultaneously transferred to light shielded areas of the sensor. The charges are then shifted out of the light shielded areas of the sensor and read out.