These meshes are placed slightly under the full terrain and aid mods like Enhanced Volumetric Lighting and Shadows and face the opposite direction and set the cast shadow flag, which is currently not possible on terrain in the active exterior cells or has no effect when done on terrain LOD meshes.
In order to enable the generation and automatic placement of the meshes, check Terrain underside in the advanced mode. The underside meshes generation is automatically checked if the SKSE DLL (DVLaSS.dll or EVLaS.dll) is detected in the load order.
In addition consider to uncomment (remove the ;) DoubleSidedTextureMask=mountain,mtn in ..\DynDOLOD\Edit Scripts\DynDOLOD\DynDOLOD_[GAME MODE].ini, so that the object LOD for mountains (the rocks) blocks sun rays as well.
Height - sets the negative distance to the actual terrain. Lower numbers mean better blocking at a higher risk of the terrain underside mesh visually peaking through the actual terrain or terrain LOD.
To test different Quality and Height settings or to only update the terrain underside meshes for an already existing DynDOLOD output installation, keep the current DynDOLOD output installed. Start DynDOLOD as usual with the existing DynDOLOD plugins and switch to the Advanced mode. Unchecked all other options but the Terrain underside checkbox. Merge the new DynDOLOD output with the old DynDOLOD output. See Updating for more.