There is a difference between doing nothing and receiving nothing. The first is a choice you can make anywhere, badly, while still holding a phone. The second needs a room.
Most rest is interrupted rest. You sit down, and within ninety seconds something arrives — a notification, a thought you can act on, a small errand your hand can do while the rest of you stays still. The interruption is not the problem. The availability is.
Why twenty
Twenty minutes is long enough for the tap to stop mattering. The first five you are still in the day: the unanswered thing, the thing you said badly, the tab you left open. Somewhere in the second five the water stops being a temperature and becomes a fact.
It is also short enough that the water is still hot when you get out, which is the only hard constraint here.
The point is not to relax. The point is that nothing can reach you.
Afterwards
Nothing happens afterwards. You get out, you dry off, the day is exactly where you left it. The inbox did not grow more than it would have. Nothing collapsed.
That is worth knowing directly rather than being told. Twenty minutes of being unreachable costs nothing, and the evidence is that nothing was waiting.