Obviously we are here talking about the original tradition. In the West, "yoga" is used about the physical exercises that we have stolen from the school of hatha-yoga.
The "oscillations of the mental substance" remind me of something I only learned a few months ago: That the brain has a "default network" that is active when we are not focused on the outside world. By itself it comes up with daydreams, memories of the past, plans for the future, and what-if scenarios. In children this network is not yet fully connected, which may be why they tend to get bored. When an adult comes in a boring situation, the default network steps in with memories or daydreams of hot sex (if male) or cakes (if female). OK, that's a bit TOO simple, but it is easy to remember.
When we have to focus our attention on some task, the default network deactivates. But as we grow older, the default network runs more and more continuously. In Alzheimers patients, it even intensifies its efforts when they try to concentrate. I believe this also strongly influences our sense of time. The time spent in the default network does not register, and so our personal time seems to shrink over the years until it reaches zero and then less than zero, as parts of our lives that we once remembered are utterly forgotten.
The immediate purpose of esoteric practices is to expand time. This is in all cases done for a higher purpose, of course, but it is interesting to see that there seems to be actual neural changes going on. Perhaps one day it will be possible to gain some of the benefits of these practices by taking drugs that suppress the "oscillations of the mental substance". Though I am not sure how many would take them, since these oscillations are like tiger cubs: Cute while small.