I never even thought of it, but I have certainly had situations where I wish I could have done something similar.
If you don’t want to watch the whole thing, skip to the 3:45 mark. The audience is clapping on the 1 and 3. In a little bit the band is gonna be coming in hitting 2 and 4 hard, so awkwardness is on the horizon. Harry pads a measure by a beat to get the clapping on 2 and 4. That is next level performance skill right there. Watch the drummers reaction in the background at 3:55 
WHAT
Well shut my mouth 
One of the greatest videos ever ha.
Oh wow I guess you can’t notice it without the 1,2,3,4 number display for the beats on the video. At least I had no clue otherwise! Even with the number, I could only make out at the 3rd attempt. Starting at 3:25 would be good to follow the numbering.
@Michael_Mark, you have to start at 3:25 and then follow the numbering at the screen and see if you can look only for 1 and 3 (ignore 2 and 4 to follow too many number within short span) when clap completes. This pattern changes, after the introduction of the number 5 odd not during his play, to 2 and 4 (switch following from 1 and 3) for clap to complete.
OK cool! 
That’s incredible! I could feel the tension instantly ease after he added that 5th beat. I wonder how many in the audience noticed.
I thought so too! 
Like what difference could it have made? Maybe no one! Audience enjoyed it, I guess that’s what mattered there, I thought. But I appreciate his genius about musical nuances.
Maybe, but I’m pretty sure I would have. I felt it instantly. I might not have realized what just happened to relieve the tension, but I definitely would have noticed things were put right.
It is definitely something to behold. Just wow!
lol
Btw, I have just watched it again, now I feel there is no difference between before and after the odd beat. It stabilizes to 1 and 3 again. To me, now it sounds more like a slip and than a trick the commentator said it was!
Maybe the youtube commentator is tricking his audience??
Once was enough. Go pick on yer 
That was just insane!!
Reminds me of a little marching maneuver that everyone in the military learns. It’s a little half step that lets you get back on the correct foot. Really neat that Connick could just throw that extra beat in there on the fly.
Changing Step on the March - Change Step - Yes I remember it well
Yeah that’s pretty cool, ill have to try that sometime.
Good one. That would be me clapping on 1&3 haha. Youve probably seen this one also but thought I’d throw it in here.
https://youtu.be/pH9l8KlcuBA
Ha!
Jlo: “I’m gonna punch you in the stomach”
It would be a bruise of honor