If you thought the camcorder was scary, chew on this- Some unscrupulous people have banked on the concept and created actual glasses with pinhole camera's inside them in the center top of the frames. These cameras use a standard CMOS camera but in front of it is the translator lens/filter that makes it possible for the X-ray to work.
The glasses, like the camcorder, beam out a wide array of inferred light then the light reflecting back is processed into an image, or in this case a series of images that make a movie. That movie is then recorded by a hidden DVR (digital video recorder) wired to the glasses.
Now that you understand how that works, let's talk about how exactly this sees through clothing, after all, cloth doesn't seem to exactly reflect light effectively- but that is one of the reasons this works.
The "security hole" in this whole scenario is based on synthetic fibers, thin (but opaque) cotton and the fact that almost all bathing suits are now made of "breathable" cotton that although covers you up nicely to the naked eye, allows the UV rays of the sun to penetrate and thus gives you a perfect tan as if you weren't wearing anything at all. This was a great innovation because you could avoid getting "tan lines" and still wear clothing.
UV rays and Inferred rays penetrate these fibers practically the same way only the inferred rays bounce the light back to a camera/glasses with a filter that literally filters out everything but the inferred light itself. The inferred light same as all other light reflects back so unless you have a filter to block all but one source- inferred, you won't achieve any type of X-Ray vision. But filter all other light out and what you get is a pixilated version of an image.
The image produced is not high definition, but it's enough to make you worry if your picture was published online.
I can't show you that here due to limitations of what's allowed on this site, but you can go to YouTube and type in "how to see through clothing" and you'll have plenty of examples of just how scary this technology is.
What's happening is the IR light reflects very well from human skin. The light penetrates the clothing/bathing suit bounces off the skin and "reports back to the lens". That light is then filtered from the daylight in much the same way RealD 3D movie glasses work to filter slightly light from the screen to reveal the "hidden" offset image (why it's blurry, there are two images on the screen). If you cover one eye the screen will go back to 2D (flat images) but through a more sophisticated process.
Since the light is absorbed by the clothing itself the only holes in the image are where the fibers are. There are tens of thousands of holes in this cloth, even though it appears opaque. It's not "clear" or "High Definition" because although there are likely more holes than fiber, there is enough fiber to fool the naked eye.