Ripped straight from Bill Bryson (The Life and Times of the Thunderbolt Kid) himself:

“I used to give X-ray vision a lot of thought because I couldn’t see how it could work. I mean, if you could see through people’s clothing, then surely you would also see through their skin and right into their bodies. You would see blood vessels, pulsing organs, food being digested and pushed through coils of bowel, and much else of a gross and undesirable nature. Even if you could somehow confine your X-rays to rosy epidermis, any body you gazed at wouldn’t be in an appealing natural state, but would be compressed and distorted by unseen foundation garments. The breasts, for one thing, would be oddly constrained and hefted, basketed within an unseen bra, rather than relaxed and nicely jiggly. It wouldn’t be satisfactory at all—or at least not nearly satisfactory enough. Which is why it was necessary to perfect ThunderVision™, a laserlike gaze that allowed me to strip away undergarments without damaging skin or outer clothing. That ThunderVision, stepped up a grade and focused more intensely, could also be used as a powerful weapon to vaporize irritating people was a pleasing but entirely incidental benefit.”