The Nitro Switch Crate is a type of crate that appears throughout the series. They appear as green-colored ! crates, and show up only once per level. When attacked, they destroy all the remaining nitro crates in the level. They are needed for box completion due to the need to destroy the nitro crates.
Some levels contain nitro crates but not a nitro switch crate. In these levels, it is possible to destroy the nitro crates another way to get the box completion. Two of these levels are Crash Dash and Crash Crush, in which the nitros get destroyed when run over by the snowball. In Gone Tomorrow and Flaming Passion, Crash must shoot all of the nitros with his fruit bazooka. In Rings of Power, Crash must shoot all of the nitros, or hit them while doing a barrel roll.
In Crash Twinsanity they are initially replaced by detonator crates, but two nitro switch crates can be found in Cavern Catastrophe which explode every nitro crate nearby. In this game it turned into the shape of a nitro crate before exploding the crates.
Nitro Switch crates are normally found near the end of levels. In Crash Bandicoot 2: Cortex Strikes Back, they are usually found slightly before the level's end after the last nitro crate, but occasionally are found at the very end of the level. It is usually a sign that the end of the level is near. They can also be sometimes slightly hidden, such as in Snow Go. In Crash Bandicoot 3: Warped, Naughty Dog became less creative with the locations of nitro switch crates, almost always placing them at the very end of a level. There are a few levels, however, where it isn't right at the end of the level. These levels include Hang'em High and Ski Crazed, where it is slightly before the end of the level, and Dino Might!, where it is located in the yellow gem path, and Hot Coco where it's in the middle of the level although it is needed to properly exit it.
The reason nitro switch crates were created are so that the nitro crates can be destroyed to get box gems. However, in some levels, exploding all the nitros can have other purposes. The following are some of the other advantages of hitting nitro switch crates:
In Ski Crazed and Hot Coco, the warp sphere at the end of the level is blocked by a row of five nitro crates. Therefore, Coco must hit the nitro switch crate in order to just finish the level (although in Ski Crazed an Aku Aku mask could be sacrificed instead).
In Air Crash, the nitro switch crate is located in the area accessible via the secret warp room. Some players may find it easier to complete the death route without nitros, which is done by entering the level via the secret warp room.
In Un-Bearable, the nitro switch crate is just past the pit that leads to the secret area, and that area is much easier if Crash hits the nitro switch crate and then goes back to enter the secret area.
In Final Countdown, there is a row of nitro crates blocking a life crate. The way to break the life crate without wasting an Aku Aku mask is to reach the nitro switch crate (which is actually in that level's bonus round) and then retreat to the life crate. This same type of thing can be done in Just in Slime, where nitro crates are also blocking other crates.