The OVP (overvoltage protection) relay uses a Zener diode to act as a voltage regulator, spike suppressor. The diode allows current above a certain voltage to pass through the fuse to ground. Small spikes are just shunted larger ones blow the fuse.

The device is similar to a UPS (uninterruptable power source/whatever) for the car computors that are hooked to it.

The reason it causes so much problems (amongst those not familiar) is that the main system affected (KE-Jet) works rather well without electronic control. It looses all its enrichment schemes; most noticably cold start/warm-up enrichment.