When starting the upward movement, the piston causes the pressure to rise slightly, which will cause the suction valve to close. The gas inside the cylinder will be confined and, with the piston rising, there will be an increase in pressure and temperature due to the decrease in the volume of the gas.
The piston will continue to rise and the compression will only end when the pressure inside the cylinder reaches the opening point of the springs that until then kept the discharge valve closed.
