Here’s the situation:
We have a deep cut or something like that and want to “fill” it without destroying what’s around it. The clay brush would just work fine for that but especially if it’s a rather complex shape or probably a lot of such cuts you will have to spend quite some time working carefully on each of those.
But in fact you can make this a bit easier by using the smooth brush… yes I mean it, the smooth brush.
It may sound a bit crazy at first to use the smooth brush when it comes to detail preserving techniques but by altering it a bit even that works rather well.

All you have to do to get things like that working is to play with the “BrushMod” slider:
If you want to do what I just described above you have to move it way up to 100. (or less, that depends entirely on what exactly you want to archieve)
It won’t fill the crack completely and you will have to slightly tweak it with the clay and/or the standard smooth brush but it will make the initial step of filling deep cuts a lot easier and cleaner.

On the other hand, if you use negative values it will of course preserve the opposite, which is at least just as useful.