I differentiate 'undercutting' from 'backcutting' (next video) technically. Both are aimed at increasing the drama of the subject by adding lots more shadow.
Here we start off the undercutting (from the front) knowing we'll be able to get at other parts better from the back (ie backcutting).
As a rule, you under/backcut only as much as is needed from where you will be viewing; you want to avoid weakening the carving unnecessarily. This means you need to know the view point: High up on a wall? You'll undercut more those parts nearer the floor. Low down on a cupboard? You'll undercut the upper edges. Eye level? About equal.
So after you've been working a while (and when backcutting too), put the carving up at the level it'll live, step back and have a look.