The Smooth operator assigns smoothing groups to the faces of particle shape meshes.
Auto Smooth: When disabled, no mesh faces will share smoothing groups. When enabled, mesh faces will be assigned smoothing groups based on the angles between their adjacent faces.
Prevent indirect smoothing: re-processes smoothing groups generated by the auto-smooth function, in order to prevent indirect smoothing.
Indirect smoothing happens when adjacent faces (which do not satisfy the angle threshold condition) end up as members of the same smoothing group because they are connected by other adjacent faces which do satisfy the angle threshold condition. “Prevent indirect smoothing” attempts to rectify this problem by shuffling smoothing groups in a way that keeps properly-smoothed adjacent faces as members of the same group, while keeping indirectly-smoothed adjacent faces as members of different groups. Enabling this feature has a performance cost that will cause the auto-smooth algorithm to run slower.