What's this 13 second trick to catch cheaters I keep hearing about?

I’ve been contemplating monitoring apps, but I feel uneasy about invading someone’s privacy, even if it’s for catching cheating. Do others here have experience with these tools? Did they feel it was justified? I’m curious about whether people find it ethically okay or if it crosses a line, even if it seems effective. Would appreciate honest opinions from those who’ve used or considered such methods.