Composters allow the player to see through walls.

When a player is pushed into a one block gap with a composter, or a block pushes a player into the composter, the player is able to see through walls. In addition, it seems like pushing the composter upwards causes the player to clip through the composter, while the player enters the laying down/crouching animation as normal when a block pushes them into the one block high area inside of a composter.

The first two images are when the composter is pushed up to create a one block gap, while the second two images are when a block is pushed downwards to make the inside of the composter a one block area.