Going back to display by using your palm

I guess it's not about the light sensor but about the touchscreen. 

If the touchscreen is off, it's not working for me.

If the touchscreen is on, it's working for me.

Personally, i'm not using it as i keep the touch off except for changing some settings from the watch. Also with the touch on, i find it quicker to just press "back" few times but it's a personal opinion. 

more useful is to hold a 'back', by default it goes to main screen no matter where you are (how deeply in any menu).

its not a long press like most other watches, instead its a quick tap with palm covering the whole screen. And I do mean a quick tap.

Unfortunately doesn't work when tracking, so really just for normal operation.

Thanks a lot, got it and works with the quick tap. Strange it is not mentioned in the Garmin manual.

Long taps work just as well.

However, your palm has to be _flat_ and fully touch the screen, even in the center. If you cup the palm, it doesn't work. So for people who see a difference between short and long taps, I suspect their short taps are faster and the center of the palm touches the screen vs. long taps are slow with the palm cupped.

weird as I have just never got the long press/tap to work, but always works exceptionally well with the quick tap.

For me it just disables screen....

That is how it works if you do t enable Always on Display. 
Going back to the watchfaces is AOD behavior like on the MIPS display in the Fenix.