Progress mode

There are two modes the engine runs in: progress mode and edit mode. In progress mode, progress is saved. In edit mode, no progress is saved and you can not leave the level via flying through an exit. This way, you can be sure that what you're editing is the virginal state of the level when a new game is started.

In other words: when you play the game, you're in progress mode. When you edit the game, you're in edit mode.