וַיִּשְׁלַח֩ יִשְׂרָאֵ֨ל אֶת־יְמִינ֜וֹ וַיָּ֨שֶׁת עַל־רֹ֤אשׁ אֶפְרַ֙יִם֙ וְה֣וּא הַצָּעִ֔יר וְאֶת־שְׂמֹאל֖וֹ עַל־רֹ֣אשׁ מְנַשֶּׁ֑ה שִׂכֵּל֙ אֶת־יָדָ֔יו כִּ֥י מְנַשֶּׁ֖ה הַבְּכֽוֹר׃
Da streckte Israel seine Rechte aus und legte sie auf das Haupt Ephraims, der doch der Jüngere war, und seine Linke auf das Haupt Manasses. Er legte seine Hände mit Bedacht, wiewohl Manasse der Erstgeborene war.
Rashi on Genesis
שכל את ידיו PLACING HIS HANDS DESIGNEDLY — Understand this as the Targum renders it: אחכמינון he put wisdom into them (viz., into his hands), meaning designedly and wisely he moved his hands for this purpose, intelligently and with full knowledge, for he knew that Manasseh was the first-born and yet he did not place his right hand upon him.
Rashbam on Genesis
שכל את ידיו, as if the Torah had written שכל, both the letters ש and כ with the vowels kametz, resulting in sachal, meaning something distorted, bent out of shape.
Sforno on Genesis
שיכל את ידיא, he was quite clear, relying on his sense of touch, without being able to see exactly where his hands came to rest.