Albert Barnes Commentary 2 Chronicles 30:16

"And they stood in their place after their order, according to the law of Moses the man of God: the priests sprinkled the blood [which they received] of the hand of the Levites." — 2 Chronicles 30:16 (ASV)

After their manner - According to the Mishna, the custom was for the priests to stand in two rows extending from the altar to the outer court, where the people were assembled. As each offerer killed his lamb, the blood was caught in a basin and handed to the nearest priest, who passed it to his neighbor, and he to the next. The blood was thus conveyed to the altar, where the last priest in the row threw it at its base. While basins full of blood were passed up, empty basins were passed down in a constant succession, so that there was no pause or delay.

Which they received of the hand of the Levites - Ordinarily, the blood was received from the hand of the offerer. But the greater number of the Israelites who had come to keep the feast were involved in some ceremonial or moral defilement, and there had not been enough time for them to purify themselves (2 Chronicles 30:17). Because of this uncleanness, they did not slay their own lambs but delegated the task to the Levites.