>>28801432It's because it implies that you're seeing something that's usually hidden, something perhaps secret that's not supposed to be seen in daylight and in public so boldly.
It ties this shameless reveal with the implication of intimacy into one ultra lewd package. Back in the day, when girls still wore such things, even the tanlines of a watchstrap in summer was enough to give strange and thrilling twinges in my male brain that were not conscious surface level thoughts.
Want to make something already ecchi even more so?
Tanlines, the lewdness magnifier.
