Genesis 48:13

‘And Joseph took them both, Ephraim in his right hand towards Israel's left hand and Manasseh in his left hand, towards Israel's right hand, and brought them near to him.'

Manasseh is the eldest and should receive the blessing from the right hand in acknowledgement of his seniority. The right hand was conceived as being the most powerful, as it usually is in practise. Thus Joseph guides them towards Jacob in the right positions for the blessing. But Jacob in his dying insight is aware of something that Joseph is not aware of.