A reaction shot is a camera technique in which the scene cuts away to a person or persons to capture their reactions regarding a particular event.
To get a good idea of how reaction shots work, watch any famous award show. The Academy Awards are notorious for reaction shots. There will be several cameras trained on the nominees while the camera with the main feed is viewing the podium with the presenters. Once they get to "And the winner is...", the camera operators prepare to have the feed switched to them so they can capture and broadcast the reactions of the losing nominees upon hearing the winner's name announced.
In many cases, there will be a split-screen deal where all nominees will be on the main feed simultaneously in a small part of the screen, and when the winner is announced, all their reactions are captured.
Most people who attend award shows are good sports, and while disappointed they lost, are happy for the winner. However, there are those rare times when someone gets genuinely pissed when they lose.
| 
