i think what he does there (twice) is fret with index and slide up the neck,
then either with his thumb or with a quick flamenco fingernail flick,
hits a higher note and slides down, probably fretting with ring or pinky,
so that finally he can catch the next "real" note of the song with index.
that, or, like others have said, he could be tapping it with his thumb and sliding down with his right hand.
if you were gonna do this live, both would sound and look cool
