On Saturday, 10 April 2021 at 20:08:55 UTC+1, Roger Mills wrote:
There are actually two types of Monsoon pump - standard and universal.
The standard ones need a positive head - and enough gravity flow to
trigger the flow switch. The universal ones can work with a negative
head. They also have a pressure vessel and a pressure switch. Presumably
the pressure drops when a tap is opened, and triggers the pressure switch..

Indeed. Also, I believe the universal ones have better-rated seals, to cope with the possibility of the pump running dry. For that reason, if my current 9-year-old standard ST Monsoon ever needs replacing, I will probably replace it with the universal version.