That method doesn't necessarily seem useful because he's touching the part to make it do things that you don't want in the circuit. The parts I've used are completely non-polarized and do not care which way you install them - AC will pass through either direction. The manufacturer doesn't mark the foil end because it's not important. Old paper caps? Yes, that was important. ... edit: I'd suspect the method because you don't know how the part is wound or made. You could just be affecting the capacitance because the outer coating is closer in that area. You wouldn't know unless you removed said coating.