Notice that the sheep pass under the rod. Besides being an instrument of both offense and defense—the rod was, in effect, a two-foot club—it also functioned as a tool, under which the sheep passed. What does this picture? First of all, it pictures counting. The shepherd would count the sheep in his flock to make sure they were all present and accounted for.
It pictures something else too. As the sheep passed under the rod—a symbol of the Word of God—they would undergo a close scrutiny. The shepherd would run his rod backward or across the grain, as it were, of the wool. The rod separated the wool, allowing the shepherd to look down onto the sheep's skin. He was then able to see both the quality of the skin and of the wool.
God is illustrating that by means of His rod, He is giving us careful, close scrutiny for two reasons: One, it gives Him the opportunity to evaluate the quality of His sheep. Two, it provides a means of separation. Quality and separation are the two reasons for His scrutiny of us.
Recall Matthew 25 and the separation of the sheep and the goats. The rod aids in identifying or making sure of possession. Sheep's ears were often bored through or distinctively notched as a mark of identification. Sometimes, since the shepherds could not always see that identifying mark due to several flocks being mixed together in the pasture, they would make the sheep pass under the rod. When they did, the shepherd would flip back the ear to see the mark of possession. Again, it also gave them a chance to evaluate and determine the relative health and quality of that sheep.
We are all under the rod right now. Now is the time of our judgment (I Peter 4:17), and we are under evaluation to determine to whom we really belong: God or Satan. Who is our shepherd? The rod is a vitally important instrument for a shepherd. No good shepherd would be without one.