Here is how a zener diode works in a MAP clamp. Let's say the factory ECM uses 4.3 volts as zero intake pressure with the MAP sensor. You connect one side of the zener diode to the wire that goes to the ECM. The other side of the zener diode goes to the ground wire. In this way, with a 4.3 volt zener diode, the zener diode will regulate the voltage on the wire to 4.3 volts by allowing any voltage over 4.3 volts to dump to ground. Since the MAP sensor works with 5 volts max, it does not have to dump much voltage to maintain the volt rating of the zener diode. It does not cause a short and so it does not cause any excessive load on the ECM.