If it's actually the headstock they are grabbing then they could be pulling and/or pushing on the neck to get a variation in pitch. Like using a tremolo bar but more subtle. More like bending actually, but instead of bending a note they are bending the whole pull-off/hammer-on lick. This is the most likely scenario.
Also, if it's a player who is NOT using a Floyd rose tremolo with locking nut, he could be pushing on the string behind the nut, again to cause a change in pitch.
Another trick could be that they are actually fretting a note on the string where they are performing the lick to change pitch of the open string...if the lick involves pulling-off to an open string. But if they're grabbing the actual headstock that's probably not the case.
Hope that helps. :D
-Tom
