Yanmar Excavator, changing from Rock breaker to 2-way circuit.

On some Excavators, there is only one circuit, that for reasons of economy at production level, is designed to be necessary to restrict the flow back to tank on one side of the circuit, so as to enable the circuit to be pressurised in either direction. So, the ball valve in this video is turned to the of position, to shut off the return to Tank.
If your question now is, "Why not leave the Ball valve closed, and just use one side of the Pedal or Button to use the Rock breaker?" The answer is, because it is necessary to provide a least path of resistance for the oil from the Rock breaker, to return back to Tank. Also, some Rock breakers can be damaged, if the flow of oil is reversed. It's a bit more involved than that, but that is the simple explanation.
Thanks for watching.