ive seen guitar players do this little trick where they are soloing and they take their picking hand and go up on the headstock and so something, not vibrating the open string, but doing something that i cant quite figure out. ive seen john frusciante do it and im wondering if somebody knows...?

thank you
It might be that they are using their picking hand to dampen the strings, while their fretting hand does a tapping hammer on/pull off thing. I'm pretty sure that Steve Vai does it in the tender surrender video.
they could be pushing on the string behind the nut to make he string go sharp or pulling the headstock to make it go flat.
looks to me like he picked the string between the 5th fret or so and the nut, to produce a high pitched note. dunno how it was picked up that well by the pickups tho, unless he did a harmonic.
matt bellamy from Muse often strikes the string near the tuners. you can get a tone out of it, and with distortion it sound great. It's on the Glastonbury DVD if you have it...cant remember which song though