From Quake Wiki
This entity is used like a camera in Quake, shown only when you exit the level with a trigger_changelevel brush entity. This is the "intermission" you see, usually with a nice view of a graphical highlight of the level before the next level loads, or while text is written across the screen (that text has to be hard coded into the QuakeC). Simply place the entity in your level at the place you want the CAMERA to be positioned, and give the "mangle" keyvalue three values in the form of xxx yyy zzz, where each value is separated by a single space. xxx refers to the pitch of the camera angle. Positive values point the camera down, negative values point it up. yyy refers to the compass heading of the camera. zzz refers to the "roll" of the camera. If the camera has a positive roll value, the camera will be tilted to the right, while a negative value will tilt the camera to the left. You can download a sample map with an info_intermission entity in use by clicking here. If a level ends, and the trigger_changelevel that ends the level calls for a intermission, and there is NO info_intermission entity, Quake will simply the info_player_start entity in a similar manner, but with no pitch and roll values. You can also place several info_intermission entities in your map, and Quake will semi-randomly choose amongst them when the level ends for showing the intermission view. This entity can be ignored if you select a "no intermission" flag in the trigger_changelevel brush entity.
One easy way to determine the origin and mangle keyvalues visually is to use the game itself to "print" the values for you. You'll need to use a variant of the Fitzquake or Quakespasm source ports. Load up your map and use the noclip console command to "fly" around the level. Once you've selected a nice viewpoint, bring down the console and type "viewpos". The first set of numbers is the "origin" keyvalue and the second is the "mangle." In your map editor, add an info_intermission entity and use these values.