The smaller player hitbox when flying with elytra, makes it possible for players to enter holes of 1 block height

When flying with elytra, the players hitbox transforms to have 1 block height. 

That makes it possible for players to enter smaller holes than otherwise possible.

When inside a 1 block high hole, the player is no longer flying  but the player model is lying down using the animation from flying (it can be seen both in 3rd person and inventory).