The HSI/EHSI course pointer (CP) value is supposed to be fed to your GPS.
There are many installations of the GNSx30 where this connection was not made. One avionics chap was most suprised when I told him about it
If present, this then enables the HSI CP setting to instantly transfer to the GPS.
What I find is this:
1) If the GPS/NAV switch is in GPS, and no flight plan is active, the
EHSI CP is held at 360 and cannot be moved.
2) If the GPS/NAV switch is in GPS, and a flight plan is active, the
EHSI CP is held at the current leg's track and cannot be moved.
3) If the GPS/NAV switch is in GPS, and the GPS is switched to the OBS mode, the
EHSI CP can be moved and its value is copied to the GPS OBS track value. This is really handy.
To fly holds as discussed, you never need to switch the GPS/NAV switch to NAV.
But if you do switch the GPS/NAV switch to NAV (e.g. because you want the HSI for flying an ILS with) then you can turn the HSI CP but it no longer transfers to the GPS. You have to use the knob on the GPS to set the OBS value from then on. I use this sequence to fly an ILS, which is obviously flown in NAV mode, but I set a DCT to the airport and set the OBS to the ILS LOC track so I have a secondary reference for the lateral path.
I would think some of the above is installation dependent but it is probably standard for late-1990s avionics such as
this.