cherrybim:
It seems that this minhag is still followed.
Not so much anymore. I had a rebbe who, when he went to the Mir they made him trim down his sideburns because they went past the bone.
Nowadays there are plenty of Mirs with beards.
yankdownunder:
but the Mustache needs to be trim.
This is like the Arizal who says you can trim your mustache as much as it interfere’s with eating, because swallowing the hair is a sakana.
walton157:
I don’t understand the idea 100% but there is some type of kabbalistic correlation between hair growth and activity.
YitzchokYisrael:
Yeah, I think that’s the name of the sefer too. I’ve vaguely heard something about conduits for bracha, but I don’t really understand any of this. Alot of people are also makpid not to let hair from the beard that falls out, get thrown away. Usually they put it in a sefer.