The blow/6 G is continuous. The trill is between blow/3 and blow/4 (G<->C). The 5 hole is always blocked. It's played on the IV chord of the progression, C in key of G, making for a trilled C chord, i.e. G steady on top, G/C trilling below, giving alternating octave and 5th intervals. (A very distinctive sound for a trill.)
Mechanically, the 6 hole must receive continuous air, the 5 hole must always be blocked. The 3 and 4 holes alternate.
What's the embouchure and tongue position? Clearly the tongue must block the 4; and the right side of the embouchure must remain fixed. How then does he go (block 3, open 4), (open 3, block 4)? Does the man have a forked tongue? Two tongues?