How is "Holech Rachil Megaleh Sod"?
R. Yonah: If you see that one retells words of disgrace and anger that he heard, know that he reveals secrets, so do not reveal your secrets to him. Do not say that if you give it to him as a secret, he will not reveal it. One who does not guard from Rechilus will not be careful to conceal a secret.
Malbim: One who tells Rechilus, his nature is to reveal his friend's secret. A secret is like a deposit given to the Ru'ach. Just like one does not deposit with one who is not trustworthy, one should not deposit a secret with one who is not trustworthy.
Here it says "Holech Rachil Megaleh Sod." It says below "Goleh Sod Holech Rachil" (20:19)!
R. Yonah: These are similar 1 .
Malbim: There it teaches the other direction. If you see one who reveals secrets, know that his nature is to tell Rechilus.
What do we learn from "v'Ne'eman Ru'ach Mechaseh Davar"?
R. Yonah: This gives the reason. Concealing a secret is only via a Ne'eman Ru'ach, i.e. one who is Mechaseh Davar, i.e. something improper to reveal it, even if he was not told to conceal it.
Malbim: (a) One who tells Rechilus, you should know that he is not trustworthy in his Ru'ach. (b) A Ne'eman Ru'ach, not only does he not reveal secrets - he even strives to cover the matter and hide it. All the more so he will not go to reveal and publicize his friend's affairs!