Charles Ellicott Commentary Leviticus 27:32

"And all the tithe of the herd or the flock, whatsoever passeth under the rod, the tenth shall be holy unto Jehovah." — Leviticus 27:32 (ASV)

Whatsoever passeth under the rod. —That is, for the purpose of counting and tithing them. The manner in which this was done is described by the Jewish canonists as follows: “The owner is to gather all his lambs or all his calves into the fold and make a little door to it, so that two cannot go out at once. He is to place their dams outside. As they bleat, the lambs hear their voice and go out of the fold to meet them, as it is said, ‘whatsoever passeth under the rod’ (Leviticus 27:32), since it must pass on its own, and not be brought out by his hand.

And as they come out of the fold one after another, he counts them with the rod, one, two, three, etc., and the tenth which comes out, whether it is male or female, whether it is perfect or blemished, he marks it with a red mark and says, ‘This is the tithe.’” It is to this custom that the prophet alludes when he says, “I will cause you to pass under the rod, and I will bring you into the bond of the covenant” (Ezekiel 20:37), that is, He will once more claim you, as you are marked as belonging to the Lord.