My understanding is that this command will (roughly) perform the following tasks for a given voxel v in filled.mgz with intensity 255:
1) Determine whether there are any other voxels of intensity 255 in the 3x3x3 cube centered at v.
2) If so, then further determine whether none of these share a face with v.
3) If so, then let S be the set of all such voxels, together with v itself, and identify an "optimal" collection of the remaining voxels in the 3x3x3 cube to add to S so that every voxel in S shares a a face with at least one other voxel in S, where "optimal" means that the added voxels should have intensities as close as possible to 255 in norm.mgz.