Nothing else in fishing is like watching a fish come up through the surface to take something off the top. It is also the technique that people abandon fastest, because it is quietly demanding.
What the cup does
A popper is a floating lure with a hollowed-out face. Twitch the rod and the cup throws water forward and makes a noise — the splash and the sound are the whole attraction. It imitates something struggling on the surface, which is a very specific and very short-lived situation in nature.
The window is narrow and the pause is the lure
Topwater works at first light, at last light, on flat calm water, when the water is warm. Outside that window it is not subtle, it is simply the wrong lure.
Within it, the retrieve is: pop, then leave it absolutely still for about two seconds, then pop again. That pause will feel absurd. It is where most of the fish come from, and shortening it is why a popper produces follows instead of takes.
Do not strike at the splash
The explosion is the fish arriving, not the hook being anywhere useful. Strike then and you pull the lure out of its mouth — often clean over its head. Wait until the rod loads with weight, then lift.
Knowing this does not make it easy. Everybody misses fish for a season learning to leave that pause alone, and then it becomes the best two seconds in fishing.