He's changing the capacitance of the part by touching it. Depending on how well the device is wound, and how the outer coatings are applied, you could be changing that capacitance simply by having a spot that's slightly different in the capacitor's formation - this could give you a false positive, especially if you're using something with a thick shell like orange drops. 99% of the parts you use are probably going to register fine with this method, but mission-critical stuff would need to be qualified with a more scientific method, or marked by the OEM for foil/film ends. You're never going to be touching those parts, however - so for most applications what he's doing doesn't really matter.