The mechanics are unclear, as always, but the result was a special sense of divine help and presence that enabled the leader to do God’s will. In very few places in the Bible is the coming of the Holy Spirit described in any detail (Acts 2), but the spirit descends, fills, and empowers according to God’s will and purpose.
In this case, Azariah’s filling enabled the delivery of a word from God to King Asa. The result was a strong stand by Asa against pagan worship in his kingdom.
2 CHRONICLES 15:1 – And the Spirit of God came upon Azariah the son of Oded: