Prior to performing an operation on a specific part of the body, the surgeon anesthetizes the surgical site to minimize the pain. Similarly, since rebuke is meant to strengthen the soul, one must “anesthetize the site” so as not to cause the recipient pain.

Thus in every rebuke, one must remove any sharp criticism that can cause pain to the recipient.

Igros Kodesh of the Previous Rebbe, Vol. 2, p. 476