i think hes telling someone not to raise the volume of his mike. because he's done that motion before with out shaking his finger in the beginning to mean that they should raise the volume so thats my guess
3 years late but that’s the universal musician hand signal to the sound guy to bring up his monitors. Maybe you’ve seen them before as speakers on stage pointed back at the band but bigger acts use the very expensive in-ears because they’re much more efficient. It’s how the band can hear themselves and each other. Playing without monitors or a faulty one is bad news…