In the scene in The Avengers where Captain America hits the punching bag in the gym, you may not notice that the bag bursts even before his fist makes contact. If you freeze the frames, you will notice the sand is already bursting out of the bag even as Cap begins to make his punch. The next image below show more sand bursting out but Cap's fist isn't even touching the bag yet.
Such advanced reaction effects make the action appear faster than it really is. It works well if the film or video is at normal viewing speed, but with today's digital technology, people can easily slow down the moving frames and freeze them to see what's really going on in the scene and what effects have been applied.