What is a universal (negative head) shower pump?

A universal (negative head) is suitable for boosting water pressure in any gravity fed system.  These pumps are activated by a drop in pressure in the system caused by an outlet being opened, such as tap or shower being turned on.

Positive head pumps are activated when at least 2 litres per minute passes through the pump, and are therefore activated by flow rather than pressure. All outlets that are to be pumped must receive the minimum flow of 2 litres per minute for a positive head pump to be used.

So in systems with little (less than 2 litres per minute) or no natural flow, a universal (negative head) pump will be required.