He holds his hand in front of the lens as he pushes the shutter — for just a few milliseconds mind you — and that makes the exposure brighter since the camera is metering off the light available right as the shutter’s pushed. In this case, shutter lag is a good thing!
I experimented with this today and was very pleased with the results! This image was dark and didn’t have much contrast originally. It felt kind of “flat”. I tried holding my hand in front of the lens, but the resultant shot was overexposed. I wanted to shade the lens, but not so darkly. Next, I tried holding a white envelope up in front of the camera as I pushed the shutter button; the shade from the white envelope was just enough for this exposure you see here.