The comparison drawn between the two contrasts the way grains are crushed in between a quern's two massive millstones and the way Imam Husain SA and his Ahle Bait and As'haab were pulverized with a variety of atrocities.
This qualifies the above comparison for ạl tasẖbīh ạl balīgẖ – an “eloquent comparison” in literary terms.
An eloquent comparison is when fewer components of a simile are incorporated in a piece of writing so as to make it subtle and impressive for the reader.
The saddle quern is produced by rocking or rolling the muller using parallel motions (i.e., pushing and pulling the handstone), which forms a shape looking like a saddle. These are the most ancient and widely used type of quern-stone and were superseded around the 5th to the 4th century BC by the more efficient rotary quern. The handstones for saddle querns are generally either roughly cylindrical (not unlike a rolling pin) and used with both hands, or rough hemispheres and used with one hand. This provides a crushing motion, not a grinding action and is more suitable for crushing malted grain. It is not easy to produce flour from a saddle quern with unmalted grain. The muller is also referred to as a 'rubber' or 'mouler'.
Rotary hand quern
As the name implies, the rotary quern used circular motions to grind the material, meaning both the quern and the handstone were generally circular. The hand stone of a rotary quern is much heavier and provides the necessary weight for the grinding of unmalted grain into flour. In some cases, the grinding surfaces of the stones fit into each other, the upper stone being slightly concave and the lower one convex.
In this type the upper stone is hemispherical, or bun-shaped, with a central conical hopper to hold the grain that falls down a hole to the grinding surface. It is held in position with a pivot that fits into a central hole in the bottom stone. The upper stone also has a deep horizontal socket in its steep side in which to place the wooden peg used as a handle to rotate or oscillate the upper stone. This was the earliest type of rotary quern to appear in the British Isles. It arrived in Britain in the middle of the Iron Age (about 400-300 BC) and spread into the northern half of Ireland, probably from Scotland, some time after the 2nd century BC.