Crawling

Suffocation prevention, colloquially called crawling, is a feature that allows the player to traverse areas less than one and a half blocks high.

Entering suffocation prevention
Suffocation prevention 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 1.5 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

In these circumstances, the player will automatically switch to a suffocation prevention state. When enough space opens up above the player's head, the player will automatically exit this state and begin sneaking or standing again. When crawling, the player travels slower than normal.

Trivia

 * $$, if a player swims through a one block gap and leaves the water, the player will not suffocate but still continue to swim on land, which similar to "suffocation prevention" $$.