Puraścharaṇa and Chilla

In mantra sādhanā, the Guru may (will?) advise the student to do a puraścharaṇa of the mantra. This discipline entails repeating a mantra hundreds of thousands of times in a set time span with the objective of siddhiḥ (loosely: perfection in the mantra). The path is arduous and, from what i've heard, there arise significant obstacles in completing the discipline - the grace of the Guru is required to protect the sādhaka towards successful completion.

i've wondered about the parallels of this method with other really super-laser-hard-core-focused training in other spheres. For examples, training of commandos (whether a similar  or in music. And then i came across this interview with Śri Zākir Hussain on his own training:

When we’ve been studying for a while and it comes time when we should go out and perform, when we’ve become good enough to do that, what we’re supposed to do in north Indian music, is something called chilla. Chilla is like a coming of age.
Chilla means to go away to the place of the elders, in the forest. You’re alone. You have to survive on whatever the forest has available for you. For forty days and nights you are there alone and you play music. Now if you are a drummer or a sarangi player, just imagine for 18, 19 hours a day you are playing music, the vibrations of it, the tonal attacks of it, the frequencies — you start to hallucinate, you get into a zone, and that takes you god knows where.

There have been very few people who have been able to finish their chillas. The reason for that being if you have had not-so-great experiences in your life, they will reveal themselves to you in that manner, as if you are having a bad LSD experience. And you could fry your brain. People have had some very bad experiences.

I happened to have done my first one when I was sixteen. I was still innocent and I was lucky so I had decent experiences and I saw visions and things that I shared just with my teacher, my guru and my father, and we talked about them and how authentic they were.

 So puraścharaṇa in mantra sādhanā == Chilla in music. Interesting thing is about the 40 days that Chilla is required to be done. This number also (IMHO) comes up in puraścharaṇa.