Patient triggers a breath (1) which means the machine will deliver a preset tidal volume (2), e.g. 500mls. This is the volume set by the user which we want to deliver with each breath.
As that volume enters the lung the pressure in the lung will go up (3). The pressure climb will stop at the same point as the maximum volume delivered.
As the breath is initiated flow will go up immediately to achieve the targets set until it reaches a constant flow and then plateaus off. (4).
When that preset volume is reached, flow will stop and then, as the patient breathes out, flow will actually come out of the patient, so the flow curve dips below our time axis. (5)
As that happens the pressure in the lung will start to fall (with PEEP it would not return to zero, but would stay at whatever pressure was set) (6).
And finally volume will come out of the lung and return to zero. (7)