"Thinning behind the edge" is another way of saying "double bevel." It may or may not be important. It's mostly a question of whether the knife wedges.
Most people who freehand, establish their bevel angles by "clicking in" to the old one, whether intentionally or unintentionally; and that means the angle becomes increasingly obtuse with successive sharpening sessions. Thus, edges need occasional thinning as part of routine maintenance. As a practical matter that means taking the knife to a coarse stone every fourth or fifth sharpening and carefully establishing the desired angle, while fighting a natural tendency to allow the knife to set it.
People use the term "hollow ground" to mean a lot of different things. The least ambiguous meaning is that the edge (or face) geometry was created by holding the knife against the outside of a wheel -- and is concave to some degree. Hollow ground edges are very weak and should be re-profiled to flat or convex at the earliest opportunity.
On the other hand, hollow ground face geometry is -- in general -- neither particularly desirable nor undesirable. If it works for you on your specific knife, leave it alone as much as possible. If not, the fixes are complicated and usually involve a belt grinder.
On the other, other hand: If you have a flat bevel edges transitioning to a hollow ground face, and (a) the transition is a sharp shoulder; and (b) the knife tends to wedge, you may want to soften the transition. You can do that with a belt sander, loaded buffing wheel, finger stones, sanding pads and regular bench stones. Belt sanders and regular bench stones coarse enough to move metal quickly leave marks -- so you'll want some way of buffing the marks out if you're going to use either one of those. The other methods take more time.
But on the other, other, other hand, if you're using "hollow ground" to refer to dimples (aka kullens), it's an entirely different thing.
There's no trick to establishing a flat bevel. Just hold the desired angle as consistently as you can. However, no one's perfect and edges which are created freehand ALWAYS have some convexity. FWIW, that little bit of convexity is a good thing, making the edge stronger.