Peter, I believe (not fully sure though) that he is cleaning the top vents. ...With a cleaning fluid solution and a Q-Tip.
So that the top of the amp doesn't have brownish spots, caused by the hot tubes, and the transformer, and the hot heat sinks. ...For an impeccably 'clean' looking demonstration.
* I do that sometimes myself with my amps; inside the top cover and outside of it. ...You need a strong cleaning fluid, but not too strong as to rub the aluminum, or metal.