Crawling

Crawling is a feature that prevents the player from suffocating when inside small gaps. Crawling actually covers both sneaking, triggered when the player is in an area less than 1.8 blocks high and at least 1.5 blocks high, and a second pose, commonly referred to as crawling, which occurs when the player is in an area less than 1.5 blocks high.

Entering crawling
Crawling can be initiated in any way that causes the player's head to intersect a collidable block in such a way that the area underneath the block is less than 1.8 blocks. For example:


 * Closing a trapdoor on the player
 * Using a piston to push a block into the player's head
 * Using an ender pearl to teleport into a sufficiently small gap
 * Exiting swimming mode in a one-block-high space
 * Exiting elytra mode in a one-block-high space
 * Having a tree grow over a player
 * Having a shulker box push you down
 * Getting off a pig with a saddle if it's under a one-block-high space
 * Having a boat land on your head. Honey blocks make this easier to replicate
 * In Bedrock Edition it is only possible to enter suffocation prevention by exiting swimming mode in a one-block-high space

In these circumstances, the player automatically switches to a suffocation prevention state. When enough space opens up above the player's head, the player automatically exits this state and begins standing again. When in suffocation prevention, the player travels slower than normal. The sneaking state gives the player identical dimensions to standing. In the crawling form, the player is reduced to 0.625 blocks tall, or 5/8 of a block, meaning that they can crawl over blocks 0.375 blocks tall, or 3/8 blocks tall, the height of a daylight sensor or 3 layers of snow.

Trivia

 * $$, if a player enters a 2-block-high area of water and swims into a 1-block gap then leaves the water, the player does not suffocate but still continues to swim on land, which is similar to suffocation prevention $$.
 * Crawling was known simply as suffocation prevention as the technical behavior implied that before the crawling name were official.