Before we undercut anything we need to be sure we know where the edges are and how they move in space so, first, the edges have to be 'set in'. But where exactly are the edges?
Well, of course, the edges are the ends of the forms, as the forms meet the surrounding space. So we have to get these 'forms', the leaves, right first. Once we undercut, that wood is lost beneath and we can't reduce the form at at edge without losing the contour.
So this is the order: Forms > Edges > Undercutting
That order. Undercutting is last in any carving, after you are really sure the edge lie exactly where you want them.
Undercutting will bring the whole cluster of leaves and acorns into sharp focus and give it some drama - there really isn't a better word). Another general principle is to undercut only as much as you need to give the effect you want, and without over weakening the carving.
If you put 'undercutting' into the search box top left, you'll find the technique demonstrated in quite a few other projects on Woodcarving Workshops.