This is a method that my mentor, Mr. Agajan uses to track which students are paying attention to key vocabulary in an assigned text that will be read out loud. First, before any reading is begun, the key vocabulary is introduced. There will be like five or six words. Let's say that one of the words is 'vanish.' Whether the book is read out loud to them via an audio CD in a boom box or another student reading the text, every time they hear 'vanish,' they will quickly raise their hand and drop it.
Another option is that instead of raising and dropping one's hand, they snap their fingers. I would recommend against having students snap their fingers. Some students cannot control themselves. It appears that those students get too much joy out of simply hearing themselves snap their fingers. It becomes a distraction insofar as those students will snap their fingers multiple times or not always snap their fingers when they are supposed to.
In this case, the point of raising one's hand or snapping one's fingers seems to merely show that students identify a key vocabulary word in the text. As such, the meaning of these vocabulary words should be discussed prior to doing the reading. Otherwise, those words will be literally meaningless to more students than they should be.