Of all of Jacob's grandsons, he favored Joseph's eldest two the most. Whether prophetically or in supplication, Israel lifted Ephraim and Manasseh to the same position that their uncles held. He made them fathers of tribes. He claimed them as his own sons (Genesis 48:5).
Jacob had already made Joseph promise that he would take his body back to Canaan for burial, now he began his last words to his family by blessing Ephraim and Manasseh. Reminiscent of his own elevation over his older brother Esau, Jacob gave a position of favor of Ephraim, though he was actually the younger of the two brothers. In partial fruition of that blessing, Ephraim did become more prominent than Manasseh, even dominating to the point that all 10 northern Jewish tribes came to be known as Ephraim after the kingdom was divided (following Solomon’s death).
There are several things that make this seemingly minor event significant.
First, it is a record of some of the last words of one of the most important human patriarchs of our faith. Of course, last words become significant simply because they are last. And, Joseph knew that Jacob was on a sick bed, if not his death bed.
Secondly, the kindhearted wisdom, spirituality and maturity of old Israel is so important (in contrast with his roguish youth) in establishing a better legacy.
Lastly, chronologically this is an early section of eschatological Scripture. The nature of prophecy in the Word of God is sometimes rather enigmatic. How did these prophets know what they knew? That God revealed the future to them is clear enough, but how He did it is not necessarily as apparent. In this case, Jacob was even blind (Genesis 48:10) and yet he crossed his arms and laid his right hand on the preferred grandson (Ephraim) "wittingly" (Genesis 48:14).
Speaking from experience as a preacher of the gospel, I can't really explain how I know that I'm preaching what God wants me to preach, but I have confidence that I am. I can't count the number of times that people have told me that God gave them just what they needed through a sermon that I delivered. Yet, I was unaware of any special direction apart from a thought and a burden. I know God is real and present. I know that He called me to preach. I preach in obedience to His command. By faith I assume that somehow He will direct me and use me. But I don't have visions, dreams, trances or any other kind of paranormal revelations. I do have a good ol' black book that I use as my resource and I am led by the Spirit. But, Jacob didn't have a Bible. Neither did Abel, Enoch, Noah, Abraham have a Bible to use. Today it is no surprise when God uses the proclamation of His infallible Scriptures. But, how did those ancient men know what to say? Regardless... they did!