"Power Beyond" is a way to add additional control valves to an open centre hydraulic system, which requires valves to be configured in series.
The last valve in the chain is usually the 3ph, which has an internal "return" line and because it is last, requires no PB port to pass fluid to the next valve.
All upstream valves require 3 lines -- the supply from the pump or previous valve (often labelled "P" for pump), the PB line to the next valve in the series and a return line to the fluid reservoir (often labelled "T" for tank). Banks of valves (eg: FEL joystick) generally only have 3 external ports and the other connections are internal to the ganged block. Rear remotes are often configured to be ganged like this, so you still only need 3 hoses no matter if you have 1, 2 or 3 rear remote outlets.
Note I say "supply" rather than the term "high pressure" used by Oldandtired. A Closed Centre system such as used by John Deere, has a high pressure line, but the supply line on an OC system is normally at very low pressure. The pressure in that line only rises when a function is used, and only the section from the pump to that particular control valve. But he is correct that the flow is diverted to the function in use. (note that Closed Center systems have no PB port and only need 2 lines per control valve).
The fluid required to move a cylinder flows from the pump, through the open centers of any upstream valves, to the control valve spool being operated. That spool closes off the flow through the PB port and diverts it to the cylinder, causing it to move. When it moves, fluid from the opposite end of the cylinder is pushed out and returns to the tank through the return line of the control valve. As the load increases, the pressure on the supply line increases. If the load is too heavy or the end of the cylinder travel is reached, the control valve's internal relief valve will shunt excess fluid to the tank through the return line.
Note that this means the maximum pressure available at ANY control will be equal to the LOWEST relief valve setting anywhere upstream in the chain. If certain functions require higher pressures than other functions, the higher pressure function should be placed earlier in the series.
It is physically possible, but a Very Bad Idea, to omit the PB port and simply chain valves using the return line to feed the next control in series. Yes, in normal operation, the return from cylinders will simply flow through the open centers of downstream valves and eventually to the tank. HOWEVER, this two-hose configuration opens the possibility of compounding relief valves. Upstream valves, lines and the pump itself could be subjected to the sum of relief valve settings if two controls were inadvertently actuated simultaneously, with catastrophic and irreparable results.
An alternative solution is a Diverter Valve (not to be confused with a diverter valve built into the tractor). This is usually an electric solenoid valve installed between an existing control valve and a hydraulic cylinder that can be used to operate a different cylinder with the control valve instead. Cheaper and simpler than an additional control valve in the series, it does not require a third hose, since it is fed from an existing spool valve. Popular for grapples: push the button and the curl/dump function is diverted to close/open the grapple instead. Could also be used to allow a single rear remote to operate both a top and a tilt cylinder.
Finally, note that specific models may be pre-piped to facilitate adding FEL and/or rear remote control valves. The logical flow of fluid will be as above, but the specific connection points may be more conveniently located. Reference should be made to the FSM and/or parts diagrams for details.
