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.