Wihtout power beyond, the waste oil from your cylinders (oil that is dumped out of the cylinder as the opposing side of the cylinder fills with oil) and the pressurized oil are combined before they exit the loader valve. This creates some additional fluid dynamics between the loader valve and other valves down stream (like the rear lift arms). For example, with the loader valve in the float position, when I use the backhoe or 3 pt hitch, the loader arms will raise.
With power beyond, the waste oil from the cylinders is dumped directly into your rear transmission, and the pressurized oil is plumbed to the rear lift arms, etc.
Converting to power beyond often requires an adapter sleeve that is installed in the end of the loader valve to separate the waste oil from the pressurized oil via an additional exit port on your valve.