Yaakov saw (Beraishis 48 (19)) that Efraim would be greater than Menashe and so insisted that his right hand should rest on Efraim. Did the placing of the hand bring about the greatness and if not why was it necessary?
According to Abarbanel, Yosef placed Menashe on Yaakov's right because:
להיותו הבכור והיד הימין היא גדולת הכח מהשמאלית ולכן היד הימין היא העקר בפעולות האדם
Since he [Menashe] was the firstborn, and the right hand is stronger than the left, therefore the right hand is the main one in all the actions of a man.
In other words, the blessing would be stronger when the right hand was used. However, Yaakov placed his right hand on Efraim beacuse:
ברצות י״י שיתברך אפרים יותר היה שלא חלה עליו יד שמאלית
God's will was that Efraim be blessed more, so it was that the left hand did not rest upon him.