Idle deload is a bug in the game's level loading system that can be used to load in fail-safe/stored checkpoints and levels. If performed correctly you will generally see your current level deload to a black void and as the pause menu opens your location should change to the next checkpoint the game has available.
Jak and Daxter: The Precursor Legacy uses a hand-crafted game engine with seamless level-loading, meaning you can travel from place to place without ever experiencing a loading screen. The game dynamically tosses out old levels as you leave a certain area through an invisible boundary, freeing memory for new levels to be loaded, without the player knowing any different. Similarly, levels are loaded from disc upon crossing a boundary leading into the level.
Some events in the game cause the current pair of loaded and active levels to be backed up - the four known events that cause this are Jak dying, viewing a cutscene, loading the game, and Jak performing an idle animation, which occurs after being inactive in a grounded state for 30 seconds. By interrupting the idle animation while the animation is still being loaded from the disc, it is possible to trigger a fail-safe which restores the levels in memory to the backed up state, deloading any levels not present in the backup list and loading any that were. Additionally, if the active level that is restored is different upon triggering the fail-safe, then Jak will be assigned the checkpoint in the active level that is geographically closest to his current position.
This is advantageous because it allows the player to store an active level of their choosing, then make it inactive, and trigger the fail-safe from a location that is closer to a later checkpoint in the level than the one they had access to normally, allowing them to bypass certain requirements such as defeating a boss or unlocking the zoomer.
While most actions can be used to interrupt the idle animation and trigger the fail-safe, they require quite precise timing. Instead runners prefer to use a Pause Buffer when the idle animation is about to occur, as it gives a window of roughly 2.5 seconds to time the input, as opposed to roughly 11 frames with other actions. In some cases such a pause is required to allow the game time to load the stored levels.
The idle deload was discovered by headstrong1290 on accident in February 2016. Blahpy figured out how to reproduce it a few days later. Idle deload's first useful application for speedrunning was discovered by Boomer and yankee027 while accidentally doing Klaww Skip.