How to do the Glitch
This glitch occurs when, after completing the bonus round, Crash dies before the game has finished adding all of the crates broken in the bonus round to the level's box total. The game will continue adding the crates to the total while it takes Crash back to the last checkpoint, or to the beginning of the level if no checkpoint has been hit. This means that any crates that were broken between the checkpoint and the bonus round will still be counted in the crate total, even though they shouldn't be. Such crates will still be intact, and can be broken again meaning that they will be counted twice in the crate count. But if the player dies normally after performing the glitch but before breaking any other checkpoint, the game will remember the true amount of crates.
This is only possible in a few levels, because in most levels, the game will be finished adding all of the crates to the level's total before the bonus round platform returns to the main level. It is possible in the following seven levels: Snow Biz, Diggin' It, Cold Hard Crash, Bee-Having, Night Fight, Sphynxinator and Flaming Passion.
In Bee-Having, it is a bit tricky to perform this glitch because if only the fifteen non-nitro crates from the bonus round are broken, then they will be added to the level's crate count before Crash has a chance to die. However, if Crash enters the bonus round with two Aku Aku masks, then he can explode nitro crates in the bonus round so that the bonus round platform will return to the main level before all of the crates are totaled. Then Crash can fall into the pit behind the bonus round platform, and this glitch will occur. In Diggin' It and Night Fight, it's even trickier to perform this glitch than in Bee Having.
In order to get a crate gem on any level, Crash must break exactly the number of crates that are in the level. Therefore, with this glitch, it is possible to break all of the crates and not get the gem, since the game will think too many have been broken. Also, it is possible to leave some crates intact and get the gem. Therefore, it is possible to get the crates gem in Cold Hard Crash without taking the death route.