This is handy if you're replacing the K bike instrument cluster with an aftermarket cluster but still want to retain the "start in neutral" capability of the gear position indicator circuitry inside the stock instrument cluster or possibly illuminate the neutral indicator on your aftermarket unit. (Of course you'll need to have a well-functioning gear position switch on the transmission.)
It's actually quire simple. The way it works is that all three yellow/(other color) wires are grounded when the bike is in neutral. When this occurs, the gear indicator circuitry in the instrument cluster sends 12v+ via the black/green wire to the starter button. (The starter button also gets power when the clutch lever is pulled in and the clutch switch is activated.)
All you need to do is install three relays in series as shown in the diagram below.
In the diagram I've used standard Bosch relay terminal designations:
85: Ground relay trigger
86: 12V+ relay trigger
87: Output
30:Input
When all three of the relays are grounded by the three yellow/(other color) wires, power flows from the switched 12V+ power source at the top to the starter button at the bottom.
You can use standard Bosch style relays but those are rather bulky and expensive so if I were going to do this I'd do it with
Waterproof Mini Relays