He's picking up on all their little traits. Crossing his fingers - what good will it do, really, crossing fingers as the sky burns? It should work, anyway; even for a device put together on the fly, it should work, if the Martha-clone was telling the truth about what's in the gas. He's good at putting together things on the fly. And it's only a simple atmospheric converter, nothing that special, nothing that complicated.
He fires, and the clouds gather, and he crosses his fingers anyway. This reminds him too much of another button, a thousand galaxies away, another planet under threat, another fire. The flames sweep across the sky, a rolling golden ceiling to the familiar London skyline. From above, he knows, the globe will be encompassed. This, he's seen before, for himself, leaning broken in the TARDIS doors with ash and blood on his cravat as Gallifrey burned. The continents devoured in the conflagration, the atmosphere giving fuel to his destruction.
Standing below, now, he knows what they felt like. How they must have watched as the glass dome cracked and shattered under the heat, how the screams of Dalek and Time Lord combined in the ruin. But he was the only one who could have pressed that button - now, and then, and again in just a few moments.
As the sky clears into a brilliant blue and the gilding on the spire of Big Ben shines in the sunlight, he uncrosses his fingers. Hope paid off. On to the next task; and this time crossing fingers will be no use. Crossing fingers will never be any use again.
There's a storm coming for the Sontarans, and they will burn.