In a nut shell:
Open center hyd. systems are flowing oil all the time during operation.
a spool directs the oil to do work when actuated, and the pressure rises,
otherwise it is just circulating oil under little pressure.
A closed center system uses a variable displacement pump.
When a spool is moved it causes the pump to pump oil to do the work.
The parasitic draw of a open system is minimal when the oil is just circulating with no pressure to speak of.
