Here's the way to look at the point about switching the black wire. Imagine you want to use water to spin a paddle wheel. You take a hose, run it out to the paddle wheel in a closed box, and run another hose to drain the water. You want to be able to control the paddle wheel with a faucet. You could put the faucet in the hose that delivered the water, or in the hose that drained the water- either one would work. Let's say the hoses were running through your house. If you put the faucet in the return hose, there would be water pressure all the time in the delivery hose, the paddle wheel box and the return hose, right up to the faucet. It any portion of the circuit sprung a leak, right up to the faucet, your house would flood. But if you put the faucet in the supply hose, there's no water pressure in the supply hose (after the faucet, of course), the paddle wheel box or the return hose unless the faucet is turned on. There's much less chance of flooding. Switching back to electricity, the water pressure is an energized circuit, and a leak is a ground or line fault, possibly through you. The faucet is the switch, and the black wire is the supply hose. The smaller the portion of energized circuit, generally the better off you are. Which is why almost anything with a switch has a polarized plug, to make sure the switched line is the black or hot wire.