Jamieson, Fausset and Brown on 2 Kings 10:15-18

15-18. Jehonadab the son of Rechab--(See 1Ch 2:55). A person who, from his piety and simple primitive manner of life (Jr 35:1-19), was highly esteemed, and possessed great influence in the country. Jehu saw in a moment the advantage that his cause would gain from the friendship and countenance of this venerable man in the eyes of the people, and accordingly paid him the distinguished attention of inviting him to a seat in his chariot.

give me thine hand--not simply to aid him in getting up, but for a far more significant and important purpose--the giving, or rather joining hands, being the recognized mode of striking a league or covenant, as well as of testifying fealty to a new sovereign; accordingly, it is said, "he [Jehonadab] gave him [Jehu] his hand."

2Ki 10:18-29. He Destroys the Worshippers of Baal.