it is a little valve which controls your idle below 1200 rpm and sets the amount of air needed to start and control your idle i.e if your car is warm it will be 75% ish out but if it cold it will wind in upto 75% ish. This is controled by the ecu off your water, air sensors,map and maf sensors. if it is sticking it will not be able to adjust right so consently trying to compensate so giving you a un even idle and poor start from cold but clears when the rev are taken above 1200rpm
hope this helps a bit
o cranks sensor just break down sometimes you can tapp them and it works for a bit then breaks again as it a magnet and wire wrapped around it.