What did Ya'akov mean when he said that Yosef's 'bow' was firmly emplaced and that his arms were gilded"?
Rashi and Seforno: He meant that Yosef became strong, 1 when Pharaoh accepted him and placed the golden ring on his finger.
Rashi (in Pasuk 26, citing Sotah, 36b): He was referring to Yosef's strength 2 of character, which he displayed by overcoming his Yeitzer- ha'Ra, when confronted by the wife of Potifera.
Targum Onkelos: Yosef's prophetic dreams materialized because he kept the Torah in secret, and because he trusted implicitly in Hashem, gold was placed on his arm and he inherited kingship.
Rashbam: Ya'akov is describing Yosef's strength by comparing him to an archer, who bends his arms 5 to draw the string of an extremely tough bow, which shoots arrow to a great distance, but whose string only a strong man is capable of drawing.
Which he refers to as a bow, because seed shoots like an arrow.
Riva: He stuck his fingers in the ground to pain himself and overcome his Yetzer. Targum Yonasan: His Ever Tashmish, which had become hard with desire, went limp, when he decided to refrain. He overcame his Yeitzer-ha'Ra from the powerful teachings of his father. Consequently, he became a sustainer and his name was engraved on the stones of the Eifod. 4
Rashi also gives this explanation at the end of Pasuk 26. Refer also to 39:1:1:1 (See Peirush Yonasan).
What did he mean when he added "this was from the Mighty One of Ya'akov, from there he shepherded the stone of Yisrael"?
Targum Yonasan: He overcame his desire due to the sharp teachings of his father. And because of that deed, he merited to become the sustainer (of the whole of Egypt), and to be engraved together with the names of his brothers [on the stones of the Efod].
Seforno: This salvation came about through the G-d of Ya'akov, the G-d who safeguards Yisrael - like one safeguards a precious stone.
Rashbam: The victory and kingship that Yosef ultimately attained was granted to him by Hakadosh-Baruch-Hu (the shepherd or sustainer of Ya'akov), 3 and by which he was able to sustain his father and family 4 in Egypt.
Ramban: This is similar to the Berachah that Ya'akov conferred upon Efrayim and Menasheh. 5
Rashi #2 (26): He overcame his desire through seeing the image of his father. Refer to 39:12:3:1.
דזן אב ובניִן: למה זו סיבת הברכה?
מהר"ל (נתיבות עולם, נתיב התורה פ"ח עמ' לו, ד"ה ובפרק): יוסף זכה לברכה עליונה (ברכות שמים וכו') כי השפיע השפעה עליונה לכל ישראל.
מה הפרוש "אבן ישראל"?
מהר"ל (נר מצוה עמ' יט, ד"ה ובמדרש): מידת יעקב שהוא היה קדוש ונבדל [מהחומר- רוחני], וזו מעלת האבן, כי החומר מתפעל [-נפעל על ידי אחרים] והנבדל לא מתפעל, והאבן החזקה פועלת באחר [ואיננה נפעלת על ידי אחרים]. 1
עי' במהר"ל (נתיבות עולם, נתיב התורה פי"ג עמ' נד ד"ה ובפ"ק) שם ביאר את הניגוד בין האבן לחומר.