Eagletree OSD pro aux 1 & aux 2 inputs

[Sgt Bigglesworth]

Hi all,
Just setting up my Eagletree OSD with a Guardian stabliliser expander module.
I've got as far as allocating a 3 position switch to aux 1 and a rotary knob to aux 2.
I have set the receiver input selection on the OSD Pro menu to the 3 position switch setting.
This enables me to control the gain of the stabilizer via aux 2 input and a rotary knob on my transmitter.
However, the 3 position switch to access the OSD menus is baffling. Any hints and tips would be welcome.

Final point is how to test RTH. I know it works on the bench when I turn off the receiver. However, there's no spare capacity in my set up to assign a switch to RTH test.

I've yet to find out how to engage RTH via the throttle trim method. I know how to programme throttle trim to minimum on my RC system failsafe. But how does the OSD Pro "know" when the throttle trim is adjusted to minimum and engage FS. Not sure what I need to set up in the OSD menus to achieve this.

Thanks