1. You place a waterbucket while standing in front of a wall, wich has an hole one block unter your eye height. (Compare to video)
2. You start swimming Animation/ Sprinting (Control-Key by default)
3. You swim in that 1 block height hole.
4. You can remove the water an can keep digging, sprinting, etc. without starting to suffocate in the wall what you schould. 

Feels like crawling on the ground but i bet thats not what it is supposed to be.

Sorry for video quality but due the 10MB limit i had to turn down the quality a lot.