Hi everyone...i have little inquiry iN a vacuum pump with Gas ballast what is actually happening when we open that valve.Like air entered n reduced the compression n stop liquid condensing.But how?what actually going on.Please help me n also help me with this satemnt

"If the temperature in a system remains constant (an isothermal system), vapor at saturation pressure and temperature will begin to condense into its liquid phase as the system pressure is increased. Similarly, a liquid at saturation pressure and temperature will tend to flash into its vapor phase as system pressure is decreased."

if some one can explain in a better way that would be helpful