I get it now!!

Ignition 2 is the start mode and has 12 volts while cranking (brown wire).

Ignition 1 is run mode (blue wire). It has 12 volts when you let off the key when the engine starts.

The diode is basically a check valve. It only allows current to flow 1 way.

The diode gets wired from ignition 2 to ignition 1. This will allow 12 volts from the ignition 2 start (or crank) mode to flow to ignition 1 run mode. When the engine starts and you let off the key the diode (or check valve) prevents the 12 volts from ignition 1 from flowing back to ignition 2. Thus, ignition 1 always has 12 volts and the Fitech is happy.