He was playing so many notes so fast. The fact that he has the ability to play like he did on that solo I find incredible in and of itself.
I think that guitar solo is one of the greatest. I mean, it really was one of the first times that someone played THAT fast. I mean, it had been done before but, especially live, that solo is incredible. And, it inspired Eddie Van Halen to use finger tapping and we all know where that got him and the finger tapping technique.