I can’t comment on stressful training but I have an idea about peaceful, calm training.
In the Wing Chun and Taiji that I learned in my later years the focus was on being calm amidst chaos. If you look at a hurricane you would note that the eye of the hurricane is relatively calm. In my job I had the chance to look at high speed machines and the faster the RPM the more the shaft appears to be still in motion.
So if you are shining a light on a reflective tape that is stuck on a rotating shaft spinning below 10,000 RPM you can actually see the tape turning round and round. But if the shaft is spinning above 10,000 RPM it would the reflective tape would appear to be still on the same spot on the shaft even though the shaft is rotating.
The seemingly still shaft rotating at 10,000 RPM looks deceivingly harmless but if you have long hair and it gets tangled in the shaft you will be scalped faster than you can say Whoa!
I suspect that somehow the ancient masters understood this phenomena from observing nature and this has led them to come up with the principle of seeking stillness in motion and seeking motion in stillness. Of course, if you do not understand science particularly mechanical engineering you might dismiss this principle as something that is fanciful and impossible to practice but it is not.
To realize this principle you need to first learn to be calm. Then slowly you will be able to attain the amount of stillness required to manifest motion. This is not something easy to do but if you observe high level masters they have this flavor. The more intense is the stillness the lesser outer motion is required to issue power.
Learning to be calm also clears your mind of extraneous thoughts that can impede your ability to respond quickly and correctly. Water basically has no mind, yet it instinctively seeks the path of least resistance. The more you think the more you will want to resist.
Again, this seems like hocus pocus but if you train the form correctly you will be able to train your mind to be free and flowing like water. This type of intention training will in time also cause your body to react likewise.
No one ever said it is easy and this is why we keep on training daily.