it feels pleasant because it throws air back to the cords and helps them seal easily. its a great thing to do after gigs. very therapeutic

Basically it's a semi-occluded phonation, like lip-trills etc. That's probably why it feels good.

people -for example ingo titze- often say that semioccluded phonations can regulate pushing in excess, and the pressed phonation, i really dont believe in this anymore as i know i can still push/press and constrict on a semioccluded phonation (on liprolls, straw, straw on a glass of water, tongue trills, V, D, bottle, pillow, humming, ng, etc...), so as the time passed i just ended prefering simple vowels, and also vocalfry/creaking sirens as onset of whatever clean mode and pitch

@Felipe

We use the same approach here in Denmark. The effect is similar at the vocal fold level as other semi-occluded phonations. However, when phonating into water the vibrations are fed back through the tube which helps release tensions (vibration helps loose muscle tensions - you'll notice that your whole face and neck is vibrating). Also you can use warm water which have a soothing and therapeutic effect for some people, and you can have a sip of the water as well. You can also adjust the resistance with the amount of water you use (a higher resistance usually forces the client to use correct support). It's easy to do, so not much instruction is needed. And finally, it's good to make the client aware of tensions, because if he/she tense too much, the vibrations in the face (especially the cheeks) and neck will be less than when they are relaxed. So they learn how to make sound while staying relaxed, which is ultimately the goal of this approach.

So it acts as a semi-occluded phonation, but with extra benefits.