The reason for disconnecting everything is to eliminate as many possibilities as possible. Sometimes a bad component can stop the board from booting, also a bad connectors, or a bad position for a wire can also stop the board from booting. By leaving the power as the only wiring connected, the video card as the only card installed and only the CPU and the ram installed, you're keeping on the bare bare minimum needed to start the motherboard's BIOS. Touching the power switch pins together is the only way to start the motherboard with everything disconnected. So if the BIOS still doesn't show, then you know you have a problem with one of the only components left, if it does start, then something you did after was wrong, either a wire or a faulty component. If the board still doesn't start after disconnecting everything and leaving on the bare essentials, then you have to start by trying with only one stick of ram, then with the other, then if it still doesn't start, then you may have to bring your computer to a local computer store where they have spare video cards that they can use to test on your motherboard to see if it's your video card (unless you can borrow one). Then it leaves only the CPU, memory or motherboard.