Start Power Switch

Max,

The 912iS engine normally operates on a fully isolated electrical system so that problems with the airframe power system cannot affect the engine. During normal operation the engine electrical needs are supplied by alternator “A”, and alternator “B” supplies the airframe loads, each system is fully isolated from each other on both the positive and negative (ground) sides.  The start switch connects the airframe power (battery) to the engine power system so it has power to start.  Once the engine is running and alternator is putting out power, the start power switch should be turned off so that the engine electrical system operates fully isolated.  

The start switch is two pole, and it switches both the positive and negative of the airframe power bus to the engine electrical system when it is closed.  If you leave it closed during flight you run the risk of a short circuit or other overload of the airframe power bus affecting the engine electrical system.  

The engine electrical system is supplied power in the following ways:

1. During startup, the battery provides power through the start power switch.

2. When the engine starts, it is initially powered by alternator B. (Start power can now be turned off).

3.  When approximately 2500 RPM is reached for a few seconds, alternator A takes over engine power and alternator B is connected to the airframe power bus and charges the battery.

4. If alternator A fails, the engine will take alternator B and the airframe bus will no longer be charged.

5. If alternator B then also fails, the engine will stop.  You must then engage the emergency power switch (connects the battery to the engine) and restart the engine.  You will now be running the engine electrical loads, (including the fuel pumps) on battery power, so you would land as soon as possible.  You would also turn off all non essential electrical loads to maximize the remaining flight time.  

" ... and what could happen if I eventually forget to toggle it off. "

The  Engine uses the START POWER to start the engine.
Initially, the "B" Generator takes over powering the Engine and Start Power is internally disconnected and no longer needed.
Once the engine reaches ~ 2400 rpm for a few seconds, The ECU Bus, including the ignitions and the fuel pumps transfer to the "A" Gen for the remainder of the flight, and the B Gen transfers to powering the airframe loads and charging the battery.

If you do not turn OFF the Start Power switch, Nothing adverse happens; So why bother?
Remember that the A Generator is powering the Fuel pumps.
In the event of a crash; when the engine stops, the fuel pumps lose power and also stop.
If the Start Power Switch was still ON;  The engine would revert to the Start Power supply and the ECU would remain powered and the fuel pumps would continue to pump.
Fuel pumps spraying fuel over a hot engine with powered wires sparking from the crash will produce a very bad result.
If the Start Power Switch is OFF The engine and fuel pumps become non-powered immediately.
It will still be a very bad day but at least missing the inferno.

So, you can leave the Start Power Switch ON as long as you don't crash.
It is very slim but a real safety issue.

The start power switch should be momentary, so it can't be left on once you let go of it. I have mine next to the starter pushbutton, so I can hold the start power switch on with one finger and then press the starter button with my thumb. Once it starts, let go of both.