The player's body goes inside the blocks after diving into the water after flying with the elytra dealing suffocation damage in survival and suffocation effect in creative.
How to produce:
Fill a two blocks deep 2×2 area with water.
Make a two blocks high 2×2 platform of any block.
Wear the elytra wings.
Jump from a high place then fly with the elytra then go straight into the water (from air to water without stopping the elytra glading).
Leave the water and stand under the 2×2 platform that you made earlier.
Start jumping and you should notice the suffocation effect and if you switch to survival you'll get damage.
Update by [Minecraft] Mega_Spud (Jay Wells):
Video demonstration: https://youtu.be/I2Oq8aWxwzU