See the specifications of the pump below, it describes "減圧する圧力を 2 圧 （高圧 ・ 低圧）に切換えて制御する用途に最適です。", so the pump you are talking about is most likely an internal gear pump that has dual pressure settings (High and Low).

--------------------------------------------------
Note added at 35 mins (2011-10-31 01:29:54 GMT)
--------------------------------------------------

See a general discussion of internal gear pumps here:
http://www.pumpschool.com/principles/internal.htm