Was looking at a guide for the nightfall, and they recommended pushing this button when the tank was in a certain spot (didn't say which spot) and that it would deal a lot of damage. I want to know what the button does, because every time I've pushed it it's done nothing, and where the tank needs to be for it to work properly. Also, I heard that it's possible to skip the first tank entirely and just go to the second one. Is this true? If so, how do you do it?