How do potentiometric cells do not proceed through a spontaneous reaction like in a galvanic cell? It seems to me that the cell setup of a galvanic cell is pretty much the same as in a potentiometric cell. However, galvanic cells just proceed the spontaneous direction of the net redox reaction of the anodic and the cathodic species in the cell, while in a potentiometric cell, it seems that the equilibrium activity ratios of the measured solution are not to be affected. 

I've read that in a galvanic cell, current flows as to follow the natural tendency of electron flow--from the anode to the cathode. But in a potentiometric cell, it's said that the current of electrons flowing is kept to a minimum. How does a potentiometric cell does that? How does it avoid for reduction at the measured cell or oxidation at the reference electrode to happen?

P.S. I know that the question looks uncohesive, but it's because I'm really confused how these concepts occur. Please try to clear up the misconceptions that might have been apparent from my question.
First, I wouldn't classify cells as potentiometric or galvanic. 

If you take any electrochemical cell with non-zero total cell potential and connect a wire between the two electrodes it will discharge. This will happen spontaneously in the galvanic direction. 

In potentiometric measurements of electrochemical cells, what you do is a potential measurement which ideally happens at zero current(definition of a voltmeter), and practically at picoampere or lower current. Therefore you are not allowing the electrons to flow from the outside circuit and hence your cell doesn't discharge.