Okay, see the battery positive terminal is disconnected from the battery? Then the test light is connected from the positive terminal of the battery to the positive battery cable.
(Let's assume he already confirmed his test light actually works, an unknown test light might not work but we'll ignore that possibility b/c it's easy to test the test light.)
As the test light is connected in the video, any current through the battery cable must pass through the test light. Worst case, the test light will limit the current to less than 1/3 Amp, this is 12.5V*0.333A =~ 4Watts in the case of a direct short (wiring malfunction). A sleeping computer should not consume much current (power) if the system is off, less than 20mA (0.020A), otherwise a larger drain will deplete the battery in hours, as the ski sits unused.
So lets connect our test light the same way he connected his, we should expect our light will glow either not at all or very dimly at most. If our bulb glows brightly, this means there's high current, possibly a direct short even, somewhere in the circuit that would draw much more current than we'd anticipate. Under the conditions we're attempting to create, the MPEM and it's attached components should not consume enough power to cause the bulb to glow brightly. If we pressed the start button under these conditions for instance, the bulb might glow brightly but that's about all that would happen.
So let's say this happens, our bulb glows brightly. For one thing, no smoke came from the MPEM b/c our bulb limited current to just 0.333A (4 Watts), nowhere near enough to cause thermal damage.
At the same time, we witnessed way more current than we anticipated, we were anticipating 0.020A or less, our bulb should not have been lit so brightly so now we know an unanticipated load, possibly a short, is present.
What should we do? We should begin disconnecting various connectors of the wiring harness to isolate the location of the short while watching our bulb, anticipating it should be dim, not fully bright.
Then, lets say when we disconnect the VTS connector, our bulb goes dim. There's our Ah-Ha moment, now we know the short has been eliminated because the current went to near zero, our bulb is no longer glowing brightly.
So we grab our electrical schematic diagram and possibly ohmmeter and begin to isolate the various components of the failed VTS. The other option might be to replace the VTS entirely.
So the bulb doesn't take place of our electrical schematic (An electrical schematic is an electrical road map of where current is traveling from positive terminal of battery, through the wires and various circuits then back to the negative terminal of the battery), it just limits the current to a low value that cannot thermally destroy things. That's handy b/c now we're not burning and smoking things while poking around in our investigation. We know well our jet ski electric starter motor will not crank the engine with such small current available, and the gauges are unlikely to operate, so we're not attempting to go for a ride or even start the engine, we're just testing to confirm there are no shorts. The bulb should not be glowing brightly or might glow dimly at most.
I'm not sure if I strayed off subject?