Welcome, as Neemer mentioned, the tank dump does not bypass the tank. It just empties it. Most people allow the black tank to fill prior to dumping to get the best performance on the dumping evolution. Since the black tank connects to the CG sewer system, and it is possible to get sewer flies back into your onboard system, most keep the black tank dump shut when connected. Many folks with onboard washer/ dryer set ups will open the grey tank drain while doing laundry to prevent overfilling the grey tank. Then shut the dump valves when done with laundry to build volume for assisting in clearing the drain hoses after black tank dump.
The most frequently recommended dumping method is to dump the black tank first, when it is close to full, then use the black tank flushing system (if installed) till clear fluid flows through system, then dump the grey tank prior to disconnecting dump hoses.
Most also add a clear piece of pipe in their connection to the sewer, there are many versions available that connect up to your hoses to give you a sense of the cleanliness of the fluid moving through the connections.