You can stimulate your vagus nerve simply by breathing out slowly while under moderate exertion. The sensation can be very unpleasant, but it also tends to take you slightly out-of-body for a while which can help with pushing through difficult tasks.
20th century Czech runner Emil Zatopek practiced, among other things, interval training with held breath. He could have been training his parasympathetic system just as well as respiration pathways.
It's been my experience in endurance exercise is that if you are inexperienced in it, you overreact to certain signals from your body like rising CO2 or falling O2. After just small effort of a short duration you start gasping for air. Years later, in retrospect, you wonder why you did that.
Another adaptation, in high latitude outdoor runners, is the adaptation to inhaling cold, wintry air. The unbearable burning that feels like you're inhaling alcohol somehow goes away. The interesting thing is that it appears to be permanent. Even if you're out of the game for few years, that discomfort doesn't come back. Could be psychological. If you've been there and done that, you dismiss the discomfort signals and don't pay attention to them.
and then there is the ? Wim Hoffman, breathing method, which is somewhat startling and unpleasant if you do it right, but not realy horrible, and it does become routine quickly
will try the mouthfull of water exercise
