What is the meaning of "Reish v'Kalon Pore'a Musar"?
Rashi: Poverty and disgrace come to one who uncovers (goes against) Musar.
R. Yonah: This refers to a Mal'ach Rasha who exposes secrets. He annuls Musar, and his disgrace is revealed. Also poverty will befall him, for his treachery will be known, and people will not trust to send matters with him, nor do business with him. So will occur to anyone who is Pore'a Musar.
Malbim: Tochachah is verbal rebuke via intellectual proofs. Musar is afflicting powers of the Nefesh via threats of punishments. Kalon is the opposite of honor. One who guards Tochachah, and it suffices for him what they teach him via Sechel, he will be honored. One who is Pore'a Musar - he does not even fear Hashem's punishments, and totally casts off the yoke. Not only will he not be honored - he will find poverty and disgrace. He will lose his wealth, and his honor will be reversed to shame.
Why is "v'Shomer Tochachas Yechubad"?
R. Yonah: Most people do not reach the honored Midos that honor those who have them, nor to the Midah of Charitzus, about which it says "Yad Charutzim Ta'ashir" (10:4), unless they guard (comply with) rebuke.
Malbim: Refer to 13:18:1:3.
