It is because for normally closed solenoid valves, it requires much less energy for hold-in as opposed to pull-in. The STC Power Saver takes advantage of this physical phenomenon. It boosts the power supply to the solenoid valve  for a short period of time to open the valve for a short duration and then it reduces the power once the valve is opened. 

STC Model 2W200 C-P-S power saver device can be used to drive solenoid valves, especially for solenoid valves with high startup, high temperature rise and extended on time. It is useful in reducing power consumption, lowering coil temperature, and extending the life of the solenoid, increasing initial power which can speed  operator cycle rates and response time. 

Product Specifications and features:
1. Range of applications:
            H Type: AC100V-280V 50/60Hz, 10-28W 1A, Input and output are no polarity.
            L Type: AC/DC 12V-36V, AC/50Hz-60Hz, 10W-50W 2A. Input is no polarity. When output loading is AC, it is no polarity.  When output loading is DC, pin 1 connects to negative pole and pin 2 connects to positive pole.
2. Power Boost: 20%~300%(AC only), particularly useful for large capacity and high loading products.
3. Power Saver: reduce power consumption 15%~85%, can decrease coil's temperature rise greatly.
4. High power start time: 0.3~2.5 seconds
5. Switch interval time: 0.7~5 seconds  
6. Housing Material: flame retardant, quality engineering plastic
7. Electrical Connection: Standard DIN43650A connector, protection class: IP65.
8. Ambient temperature: -25C~90C.