^^^^ What
ktriebol said ^^^^
Its a weird phenomena for sure and its difficult to explain all factors and conditions completely, so I'm not gonna try. Its electrical theory/behavioral stuff involving math that can cause anyone a migraine... and I hate math. I could relate an example to help simplify a general understanding of it for most folks, but that wouldn't provide a precisely correct definition of conditions either, so its best left to remain as clear as mud at the bottom of the Mississippi.
I doubt if I understand it completely anyways.
Diodes are generally used just like you think though. They are like one-way check valves in most simple applications of them. In DC circuits they are used to block AC, from either peak (half-wave), or both peaks (full-wave) of AC sine waves. So, since they can be used to block either peak of an AC sine wave, sometimes the reason(s) for their presence in a circuit isn't very clear.
For example: my 350D has a small diode in the wiring harness that runs from the negative leg of the neutral light indicator bulb, down to the neutral light switch, where that indicator bulb gets its ground that makes it light up when the bike is in neutral. The reason(s) why the factory put a single diode in that simple ground leg for that light bulb may not be clear at first thought. Until you follow that circuit back through the Ignition switch and notice that the CDI also connects into that same neutral light circuit. So that tiny diode serves to block high frequency AC "noise" (noise = AC sine waveform can be "seen" in the circuit (by the CDI) even though AC current can not flow in that circuit, due to the opposite half of the sine waveform being blocked/converted to DC in the regulator/rectifier) existing on the ground plane half of the alternator while the motor is running. Also, think what could happen to the CDI if a specific regulator/rectifier diode shorted on the opposite sine peak... and that tiny diode on the neutral light didn't exist...
The above example is real... and I didn't figure my 350D out right away.... The previous owner had cut that diode out of the neutral light harness and taped those wires back together. I didn't realize he had done that until I started the motor up that first time after I brought it home. Repeated head scratching and muttering foul language commenced.
Diodes are built and rated for current handling capacity. There is always losses in electrical devices, so the voltage is always greater going in, than it is coming out of a diode. Generally you'll see about a 1.5v voltage drop through a diode... they can be used as such a voltage limiting device too.
If you ever wish to let the smoke all out, just overload them. They can either short or blow open when they fail.
