Brush Clipping Tool

Could someone please explain how to use the Brush Clipping tool in UE4? I know what a brush clip tool is, and I used it extensively in the original Unreal Editor (from way back, years ago), but I can’t figure out how to get it to work in UE4. Are there any video tutorials showing exactly how to use it to designate the two point that will be used to form the brush clipping line?

<Edit> Nevermind, I figured it out. Last night it wouldn’t drop the brush clip points when I hit <space bar>, but now it seems to be working.

…Too bad that if you simply “Split” an additive brush, the Editor acts as though you’ve subtracted part of the mass regardless of what you do. Why does cutting that brush put a great big hole in a completely different brush? The original brush that was cut is not even high enough to intersect that floor.