Essentially, you charge the battery pack at home, then get 30-50 miles of operation on battery pack power before the vehicle runs like a conventional hybrid car. This allows for much smaller battery packs, so even the Toyota Prius or the current Ford Fusion hybrid just needs an upgraded battery pack to achieve full PHEV capability.
Of course that’s a model that really won’t work in mass production. Most people in the sunbelt don’t have outlets near where they park their cars. In the snowbelt where the infrastructure already exists it might be fine, but our population is fleeing the snow. Down here in the sun no apartments and very few houses have an outlet near the parking, that creates a cart-horse problem, the cars aren’t useful without the outlets but there’s no reason to make the outlets without the cars. The plug-in hybrid will be a tough sell.