How it works technically
Exit-intent scripts watch the mouse position. When the cursor moves rapidly upward toward the browser's address bar or tab area, the script infers the visitor is about to leave and fires a popup. The accuracy is good but not perfect — false positives happen when visitors reach for bookmarks or tabs.
Why exit popups don't hurt conversion
The visitor was already leaving. A well-timed offer recovers some percentage of that loss; a poorly-timed one doesn't because the alternative was a bounce anyway. This is unlike entry popups, which interrupt visitors who hadn't decided to leave. Exit-intent is one of the few "interrupt" techniques that consistently lifts overall conversion.
What to offer
The visitor is leaving — whatever you offer needs to be more compelling than what they were already deciding against. A 10% discount on a $5 product is too small. A free 50-page guide on the topic they were reading is often big enough. Score your popups for friction and timing.