Genesis 27:40

ועל חרבך—The phrase is equivalent to בחרבך BY THY SWORD. The word על is sometimes used instead of the prefix ב, as (Ezekiel 33:26) “Ye stand by your sword (על חרבכם)”, which is the same as בחרבכם; (Exodus 6:26) “by their hosts (על צבאותם), which is the same as בצבאותם. AND והיה כאשר תריד AND IT SHALL CAME TO PASS THAT WHEN THOU ROVEST ABOUT — The word תריד signifies grief, pain, as (Psalms 40:3) “I am distraught (אריד) in my complaint”. It means: when Israel will transgress the Torah and you will have reason to feel aggrieved with regard to the Blessings, ופרקת עלו וגו THEN THOU SHALT PULL HIS YOKE FROM OFF THY NECK (Genesis Rabbah 67:7).

וְעַל־חַרְבְּךָ֣ תִֽחְיֶ֔ה וְאֶת־אָחִ֖יךָ תַּעֲבֹ֑ד וְהָיָה֙ כַּאֲשֶׁ֣ר תָּרִ֔יד וּפָרַקְתָּ֥ עֻלּ֖וֹ מֵעַ֥ל צַוָּארֶֽךָ׃

and by thy sword shalt thou live, and thou shalt serve thy brother; and it shall come to pass when thou shalt have the dominion, that thou shalt break his yoke from off thy neck.