Henri SELMER Paris présente Stephen Williamson 

He is using his right hand middle finger, instead of his left ring finger, to trill from G to A.

Poster, Mr. Williams has brought his right pointer finger up to the low C#/high G# left hand trill key to effect this trill over doing so with his left pinky--the usual way to do this. I've seen him do this on the opening solo to Rhapsody in Blue as well, ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-r4teoC3OnEg.html another RU-vid post of his playing. Some players add an accessory key by Stephen Fox Clarinets on the instruments to do this without having to bring the right thumb off of its support of the clarinet. It's an advanced technique for players who think that their trill is faster/better controlled with the pointer of a free right hand, than then pinky of an occupied left.

Dave Jones no he is trilling from G5 to A5. this is the famous clarinet solo in the 2nd movement of Beethoven's 6th, aka Pastorale. so the extra key would not help. but yes he does do that for the trill from G5 to G#5 in Rhapsody in Blue and the extra key would be very helpful.

Saw it too. Risky but you can practise it. I prefer my daily vademecum

My teacher at U of H has me to do the same thing and practice both ways. If there’s time, use that R hand for those trills. But otherwise , L hand RIP Jeffrey Lerner 🙏🏼

Why the Buffet H8 ? Ha ha

Beethoven 4, mvt 4

3 Sec Audition piece !

🤷🏽‍♂️