This is one of my favorite songs and I hadn't seen this performance so I watched it a few times to figure out what was going on too. I could see what he was doing, but didn't quite understand Charlie's description.
Sullivan apparently had Buddy Holly's guitar amp turned off or turned down all or most of the way (certainly lower than 50%) because they were pissed off at each other after fighting over the song choice. Holly must have realized immediately that his guitar wasn't being amplified, but couldn't do anything - you can see how pissed he looks through the first half of the song - until the guitar break. He then "speed metaled" loud enough that the vocal mic (presumably suspended from above) could pick it up a bit. And in the second half of the song, he must be strumming really hard because you can still hear it (sounding like an un-amplified electric guitar which is why I think it's being picked up by the vocal mic).
The thing is that if you listen to the studio recording of the song, it really is rather raucous (as per Sullivan's disapproval) - including the scream and frenetic strumming at the guitar break. Here though, he was putting all of his anger into it on top of that (as well as all of his love and all of his kissing).
p.s. Buddy Holly's version of "Not Fade Away" is my pick for greatest ever rock and roll song (an opinion which has sparked a few arguments) - and that was the B-side to "Oh Boy!", also one of the greatest rock and roll songs I think