Air recirculation happens when a harmonica with not side vents in the
covers (like the Goldem Melody haprs jason plays) is cupped so tightly
that when you play a note, the only way for air to move is through the
holes not in the player's mouth.
For instance, in a tightly cupped harp, if you play low draw notes, the
only place for air to come from is through the high holes, thereby
sounding the high blow notes. The hari then passes our through the high
blow reeds, into the cup inside the player's hands, and back into the
harp through the draw reeds the player is actually playing.
Air recirculation works with both blow breaths pushing air through
unplayed draw reeds and draw breaths pulling air through unplayed blow
reeds. It works low notes activating high ones and high ones activating
low.
Low notes more easily activate high ones. And the high reeds that are
one or two octaves above notes being played are the most likely to
sound.
You can hear this on some Sonny Terry records. Jason could be doing
this consciously or unconsciously - all it takes is a tight hand seal,
and no awkward adjustments to embouchure.