The suicide diode is a simple measure to protect DC-powered equipment from damage caused by reversed polarity. The suicide diode is a simple and inexpensive silicon rectifier-type diode installed across the device's power supply leads such that power of the correct polarity will not pass through it.
This MUST be combined with properly sized fuses in series with the power leads to the device, otherwise it will not work! When power is connected backwards, the suicide diode short circuits the power supply with enough current to cause the fuses to blow.
If the diode is big enough, it should effectively clamp the reverse voltage seen at the device to about 0.5 volt in the few milliseconds before the fuse pops. It should certainly be large enough to avoid becoming a friode or SED before the fuses blow.
The suicide diode is, hopefully, a standard feature in expensive mobile radio equipment to help protect it from its installer/user.