Water in to the pump should equal water out approx, So if you have say big pipes in and out but then connect a smallmicro bore somewhere on the out pipe then the water through put is drastically cut down and may cause the pump to flutter or if it has an inbuilt pressure switch, cut off and on.
You can get round this by using a T piece on the outlet side and diverting some of the water back to the resevoir tank, this lets water circulate all the time the pump is on. If you fit a small tap/valve on this returning supply you can control how much water you send up the pole.
Its called a bypass.
