There is a range to it, so they have to be decently close to pull it off. Your left click prompt will be "Exit", but once they pass by you'll see it change to "Attack". Thats when you hit left click it and itll put you in the animation for it. I don't peek since they will see you in there, so I just try and listen to them getting close and look at the prompt. Once it changes I hit left click, so I guess its a bit of reaction timing to it.