ken said this in his pitch section of course about singing while ears closed
If you mean with earplugs or fingers in your ears, it's intructive to do so, now and then, to hear and feel the sensations in your head and body. For me, it also helps prevent oversinging when using in-ears during performance. Be careful to take the plugs out periodically to prevent oversinging, which seems to be the primary risk.
Singing too loudly or with too much air, or both. If my in-ears mix during performance is wrong (my vocals to low) then I tend to oversing. If my vocal is turned up then I tend to sing more lightly which is more often correct. The same can happen if you're simply using ear plugs and singing exercises to feel the sensations in your body.
@terence i tried to practice relative pitch training while ears closed, after that my relative pitch seemed to be weaken.
I don't think you want your ears blocked when practicing to obtain better relative pitch, or some form of perfect pitch with your instrument. I have this with guitar and am getting it with voice but don't know of any shortcuts. With guitar the 'shortcut' was thousands of hours on the instrument. With voice it shaping up to be the same thing.
Not sure what you are looking to do by plugging your ears. When performing in loud bars with insufficient monitoring it helps to have at least 1 ear plug in one ear to hear your voice resonate in your head. Although not as ideal as singing in a small room where you can hear your voice bounce off the walls in front and around you without any loud interfering noises it's still better than a loud area such as a bar where theres no surface near your voice to bounce the voice back to you.
Answers
how do i learn to tune a guitar without tuner like ken did in his video of better pitch section of volume 1
Maybe someone out there has made a course?