So, I did my own testing. @DeathSweetDeath , like @SadCryingBunny, I heard a crazy ringing sound as well along with a sensation of wanting to spit/puke and feeling dizzy. That said, I felt like I was on the verge of passing out, as I knocked over my tank without realizing it in the moment. I think that the point is you pass out quickly, so these sensations wouldn't even be registered or experienced as you would lose consciousness quickly.
I just did some testing because I wasn't ready to go and wanted to dip my toes first so to speak. Also, I thought the process would be gradual as in with this method it's a gradual drift to slumber, so I thought there wouldn't be any harm cause I would just take the exit bag off if I felt my vision fading.
What I did was put on the exit bag and inflated it at 15 LPM (apparently 17 LPM on the Argon flowmeter/regulator is equivalent to 15 LPM based on what I'm reading on SaSu) without inflating the exit bag on top of my head and hyperventilating first. I didn't expect anything much to happen, but it happened really fast. Like I said, I experienced all of the above sensations. I bet that if I inflated the exit bag on top of my head and hyperventilated before pulling the exit bag down, I would've passed out and CTB-ed.
@SadCryingBunny , I think that if you do everything quick enough, you would bypass this process, as you would instantly lose consciousness. Actually, now, I don't think it's 100% painless, but it's painless in the sense that if you do it quickly enough it won't be if that makes sense. In other words, it's painful if you're conscious.