Remembrance versus forgetting Hashem

שייך לעיל במ"ש שיוהכ"פ מיוחד נגד גיד המילה כמו כן זה לעומת זה ת"ב מיוחד נגד גיד הנשה כדאיתא בזוה"ק.

This relates to what was written above, that Yom Kippur is uniquely set opposite the gid (sinew) of the milah (circumcision); correspondingly, "this against that," Tishah B'Av is uniquely set opposite the gid hanasheh (the displaced sinew), as is stated in the Zohar HaKadosh.

Building on an earlier piece, the Sefas Emes draws a parallel: as Yom Kippur (a day of holiness and connection) corresponds to the sinew of the bris milah, so its "opposite number" Tishah B'Av (a day of mourning and rupture) corresponds to the gid hanasheh, the sinew Yaakov was struck in.

וכמו שגיד הנשה מנשה פולחנא דמרי עלמא כמו כן המילה הוא בחי' הזכירה כמ"ש במ"א מזה:

And just as the gid hanasheh causes one to forget (mansheh) the service of the Master of the world, so too the milah is the aspect of remembrance, as is written elsewhere about this.

The two sinews embody opposites of memory: the gid hanasheh (rooted in the word "to forget") represents forgetting one's avodah to Hashem, while the bris milah represents the constant remembrance and bond with Hashem.

Summary: The Sefas Emes contrasts two days and two sinews as mirror images of each other — Yom Kippur and the gid of milah embody zechirah, the abiding remembrance of and bond with Hashem, while Tishah B'Av and the gid hanasheh embody the forgetting of the service of the Master of the world. Holiness is the work of remembering; the rupture of galus flows from forgetting.