As the Lord had judged the fathers in the wilderness long ago, so He would judge the children of His people. He would discipline them to sanctify them (cf. Jeremiah 33:13). It was customary for shepherds to count their sheep as they passed under their staff that they held over the doorway of the sheepfold. [source][source][source]
"Reference to those who "pass under my rod" was first an allusion to the tithe ( Leviticus 27:32). Every tenth animal that passed under a "rod" held over the sheep was separated and declared to be holy. The purification of the exile, likewise, would separate the righteous and the wicked. The "rod" also was an instrument of discipline, correction, and punishment. This was another way of communicating the purpose of the exile, which was to "purge" and purify those who rebelled against God ( Ezekiel 20:38)." [1][source]
The Lord would also bring His people under obligation to keep the terms of a covenant. This is evidently a reference to the New Covenant (cf. Ezekiel 36:25-38; Jeremiah 31:31-34). He would weed out the rebels and transgressors from among them and bring them out of the countries where they lived but would not bring them into the Promised Land (cf. Numbers 16; Deuteronomy 11:6). This probably refers to the Jews who will die during the Tribulation period (cf. Zechariah 13:8; Revelation 12:15-17) and or when the Lord returns to the earth (cf. Matthew 25:31-46). Of course, many other Jews will enter the Millennium, as the verses cited make clear. [source][source][source]
"The passage is a prophecy of future judgment upon Israel, regathered from all nations ... The issue of this judgment determines who of Israel in that day will enter kingdom blessing ( Psalm 50:1-7; Ezekiel 20:33-44; Malachi 3:2-5; Malachi 4:1-2)." [2][source]
Then His people would know that the One who did this was Yahweh. [source][source][source]