It used to be common for flashlights to have a button that, when depressed, would temporarily light the bulb. It wouldn't click into an "on" position, so when you released your thumb the light would go out. Presumably this was for signalling with morse code. I suppose the military still uses this feature, but most flashlights I see now are simply on/off and have no signalling button.

For no good reason, I saved the several "official" Boy Scout flashlights which I used when I was a lad. All of them have this "Morse" feature ... because we were all supposed to learn the code and use it.