A flying start is a technique for beginning a larp, in which the game starts immediately, for everyone, at a given moment.
Players are told come up with a situation, task or something that the character is doing, and freezing in that action/place. It can be in the middle of conversation, or a situation where the player is going somewhere to perform some action. On a given signal (usually a gas horn or something similar) the game unfreezes as if you pressed play on a paused videotape, and the game starts.
The flying start is used to get the game going simultaneously, and to get a smooth transition from off to on.
In litterature and film the term relates to the concept in medias res. In medias res or medias in res (into the middle of things) is a Latin phrase denoting the literary and artistic narrative technique wherein the relation of a story begins either at the mid-point or at the conclusion, rather than at the beginning (cf. ab ovo, ab initio), establishing setting, character, and conflict via flashback or expository conversations relating the pertinent past. The main advantage of in medias res is to open the story with dramatic action rather than exposition which sets up the characters and situation. Because it is a feature of the style in which a story is structured and is independent of the story's content, it can be employed in any narrative genre, epic poetry, novels, plays, or film. Source (http://en.wikipedia.org/wiki/In_medias_res)
Obviously so it the case with larps.