Sforno on Genesis 48:18/ספורנו על בראשית מ״ח:י״ח

1כי זה הבכור. שים נא ידך על ראשו. Once he would place his hand on the head of Menashe, Joseph felt sure that his father’s mind would follow the direction of his hand. This has been illustrated in Numbers 27,23 when Moses placed his hands on Joshua when appointing him as his successor. [if the physical contact would not be of significance, why would Moses have bothered? Ed.] The right hand is presumed to be stronger than the left, hence a blessing conveyed by means of the right hand would be a stronger, more comprehensive blessing.