As the name suggests, the twist modifier will convolve the base density values along a central axis. A slider gives you the ability to control the strength of the twist modifier.
The Inside/Out modifier translates density values from the inside to the outside, and vice versa. For example, the density value located at the center of the voxel field is translated to the outer most voxel coordinates. As the filter moves out from the center, each value is translated to coordinates that are moving in toward the center, until the outermost density values are combined and assigned to the center voxel. The result is an Inside/Outside effect.
The Gaussian blur modifier is a smoothing filter which is the result of applying a Gaussian transformation in three dimensions. The effect is similar to Gaussian blurring filters found in 2D image processing, only applied to a 3D voxel field.
The Symmetry modifier will copy density values within a voxel field across orthographic planes such that the resultant geometry is periodic and symmetrical.
The Minimum modifier is a 3D kernel-based filter which replaces each voxel density with the minimum value within its range (ie. neighborhood). The effect results in an erosion or skeletonization of the original source geometry.
The Maximum modifier is a 3D kernel-based filter which replaces each voxel density with the maximum value within its range (ie. neighborhood). The effect results in a dilation of the original source geometry.