I just saw a paper on that. I think the basic idea is that the reason you remember better from handwritten notes versus typing is that each letterform has a unique shape that you have to write down. So your fingers/hands are following along by some sort of choreographed muscle memory when you're writing stuff down, it's like a sort of dance that our hands do, tracing out all these letter forms, there's more uniqueness and complexity to it that somehow stays with us better. Compare that to typing where you're literally just doing the same action over and over again, you're just pushing buttons down. You might be able to focus more on what the professor is saying, but you're more just passively taking it in and your mind isn't as engaged in your note-taking.