The valve is extremely accurate and quiet when controlling to a set point temperature in normal operation. Because the valve measures the temperature every minute using it's two temperature probes it is very accurate at measuring the temperature in the room. The valve has 1,000 steps between the fully closed position and fully open, so it can adjust the amount of water coming into the radiator to a very high degree of accuracy, giving it unparalleled control of the heating in the room.
When a valve is first mounted on a radiator it has to 'learn' the on position, so it can work efficiently and react faster in the future. When it is in this 'learning mode', each time the valve is called to heat a room it starts to control the temperature by opening 5 of the 1,000 steps, it then waits to see if it can sense any hot water flowing through the valve body (the metal part that the water flows through), and then after another 60 seconds it opens another 5 steps etc.
When mounted on the flow side of the radiator it will normally have established the 'on' position of the valve after about 30-45 minutes, it will store this in its memory and will then control as normal to allow the radiator to warm up.
Next time it opens it remembers the position that it needs to go to and immediately starts to control around that point.
In the case of it being on the out flow of the radiator, it may reach the 'on' position, but because the water is flowing into the other end of the radiator it will not sense a temperature change, because of this it will continue on the 5 steps routine for much longer. After the first week the valve will have eventually worked out the correct 'on' position but it will take much longer for it to do this in this case.
To work out what side of the radiator is the in-flow and what is the outflow you can purchase magnetic radiator thermometers, or you can simply use your hands (WARNING: the radiators may be very hot to touch). The warmer end of the radiator nearest the valve/pipe is the inflow end and the cooler end of the radiator is the outflow end.
You can stop a valve from learning if you simply press any button on the valve whilst it is trying to warm up the room. In this case, the valve abandons the learning for that time and will wait until the next time it is called to heat a room before it tries to learn again.
Don't forget that if you ever put a valve into 'mount mode' again then it does not know if you have taken it off and put it on a different radiator so it will start it's weeks learning again.