Have been using aquatap for a couple of weeks, yes it can be partially open to restrict flow, I also still have tap on pole hose which I used before aquatap so that can be used also to restrict flow. I still use the tap for awkward windows as I haven't yet mastered turning the aquatap on/off at full stretch, mind you I am using a CLX 27 so it is probably more awkward than a stiffer carbon pole.
Neil