As you try to move from a larger diameter to a smaller the pressure increase. The fluid has to travel further in a smaller bore for the same displacement of volume in a bigger bore pipe. Basically the system follows the conservation of energy concept.
Try blowing down a 1/2 inch pipe and then a straw, you will feel the increase in pressure as the diameter narrows. Basically the system follows the conservation of energy concept, as Paul say's it is effectively a lever.
I looked at my old Uni book on the subject "Fundamentals of Momentum, Heat & Mass Transfer" by Welty, Wickes & Wilson. I realise firstly how much I have forgotten but it is a gripping read on the subject for someone but probably not for me.
