It keeps the beans from foaming up as well as keeping the skins from popping off and clogging up the vent tube. Cook according to the chart below. Soaking for 12 hours will greatly reduce the cooking time. Turn off the cooker and let it sit for 1 hour.
Unsoaked beans take the longest.
When the cooking time is up, quick-release the pressure cooker under cold water to avoid foaming or sputtering at the vent. If you do hear a sputtering sound, place the cooker in the sink and run cold water over the entire cooker to bring the pressure down quickly. Remove and clean the lid, vent, and rubber gasket. Lock the lid back in place and proceed with cooking. Be sure to check the vent tube to make sure no bean skins are stuck in it.