I should have written Pressure Drop.
The refrigerant flow moves through the metering device at a rate based on the size of the orifice and the difference in pressure across it. As the Pressure Drop reduces - so does the flow rate. This reduces the refrigerating effect and is why head pressure must be controlled to maintain a certain minimum Pressure Difference. Typically you would be OK using 100 lbs. as a minimum number.
Sorry about that - I am becoming a dullard apparently.