Rod tip dip.....
I teach the "rod tip dip" and use the 'shallow saucer' analogy. I use the same motion when I single spey. I don't have a bias one way or the other, it is simply the way I was taught.
When the rod stops at the end of the "rod tip dip," I tell the caster to imagine that his/her rod tip is frozen is a square centimeter of space behind him/her. The forward cast begins from this spot.
This analogy seems to cut back on the tendency for the hands to creep closer to the body; it also helps keep hands, rod, line, and most importantly rod tip in that magical 180 degree alignment fore and aft.
I don't teach the Underhand cast, but I fish with it extensively. With the Underhand cast, I use less "rod tip dip." With a shorter head like the Adapted line, I often get away with just a straight pull and no "rod tip dip." Not sure if this is right, but it feels good and is less tiring over a long day.