Apparently they'd all dropped acid right before going on stage, so that probably helped a bit. :)
In several interviews, Santana recalled experiencing the effects of psychedelics during the performance, but got it together for the finale. "By the time we got to 'Soul Sacrifice', I had come back from a pretty intense journey. Ultimately, I felt we had plugged in to a whole lot of hearts at Woodstock".
"The neck of the guitar started wavering like a snake, and I was like oh, uh-oh, so I just started making faces, because I was trying to keep it tame, you know, I was trying to keep it from slithering all over the place."[0]