The pictures of the baffles are accurate, but ignore the "hose spraying water" analogy. A high baffle does not choke off the air to create higher pressure inside the oral cavity. It also doesn't have any kind of venturi effect. The baffle effect is acoustical, not hydraulic.
I don't know enough about the science involved in the effect of a baffle on sound. But the reason water speeds up when a bottle neck is formed (like pinching a hose) is because water is not a compressible fluid. So when a given volume of water moving at a given speed flows into a smaller space it has to speed up because it won't be compressed.Although it isn't really a fluid flow situation, it seems that if the oscillation has enough magnitude, conditions in the mouthpiece could mimic some flow characteristics. Or maybe not.