When the pump plunger passes BDC, the inlet valve closes due to the increasing pressure in the high pressure chamber. The fuel in the high-pressure chamber can no longer escape.
As soon as the pressure in the high pressure chamber exceeds the pressure in the high pressure channel, the outlet valve opens and the fuel is forced into the high pressure channel (delivery stroke).
The pump plunger delivers fuel until TDC is reached. The pressure then drops and the outlet valve closes. The pressure on the remaining fuel is reduced. The pump plunger moves downwards.
If the pressure in the high-pressure chamber falls below the transfer pressure, the inlet valve reopens and the process starts again.