While everyone in Japan was rushing out to exchange their old flip-phones for the latest touchscreen smartphone available, there was a particular group of people who stayed faithful to their old phones — ironically, it was the unfaithful.

Japanese men who love to juggle girlfriends or have secret relationships on the side love their Fujitsu F-Series phones, and it’s due to one specific feature — a privacy option that can make your secret communications near-invisible to the untrained eye. It hides texts, calls and voicemails from numbers that are designated as private, so anybody snooping around your phone would have no idea. Japanese blogger Bakanabe tells The Wall Street Journal:

“Women may want to check my phone for strange emails or calls when I’m not around. With Fujitsu’s ‘privacy mode,’ they can’t see that information at all. The key is to give off the impression that you’re not locking your phone at all.”

The reason why this privacy mode is almost invisible is due to the extremely subtle ways that it alerts its users a private number has tried to make contact. When one of those numbers tries to get in touch, the only sign would a subtle change in color or shape of how the battery or signal levels are displayed. When the privacy mode is turned off through a combination of keys, that’s when the hidden messages or calls appear.

Just how effective have these phones been for concealing secrets? I’ll let Professor Natsuno of Keio University answer that question: “If Tiger Woods had this Japanese feature in his phone, he wouldn’t have gotten in trouble.”

[via The Wall Street Journal]