1) At least twice a day the Chazzan says Borchu es hashem hamevoroch.
2) The Kol Bo writes , when the chazzan says Borchu, he must bow down. (mekor)
3) There is no mention anywhere, for the tzibur to bow when answering (no mekor)
Boruch hashem hamevorach
4) The Mechaber & Rema write ( O ch. 113 – 3) שאין לכרוע אלא במקו שתקנו חכמיס
You may not bow down only in places where chazal instituted to do so .
5) The mogein giborim writes , people bow at borchu but we can’t find
a smach for it “ (a heter ).
How and why is it, that today everybody bows down, when answering after boruchu ?
Wouldn't it be neged halacha?
The maharashdam * (y”d 193) writes ,on minhagim that go back
a long time, and are against halacha, we must search until we find a
mekor (source). to come up with a heter. “Minhag Yisroel Torah”.
Therefore the Biur Halacha (113 3) came up with a smach (for tzibur bowing)
from a pasuk in Divrei Hayomim (1-29- 20) “Vayomer Dovid --- Borchu noh
es hashem elokeichem, Vayevorchu, --- Vayikdu VAYISHTACHVU---
Even though it’s a dochek to apply this to our case, but Minhag Yisrael Torah.
We do find Poskim , calling some minhagim , minhag shtus, or mitzvo l’vateil etc.
probably the difference between minhag shtus and M.Y. Torah depends
if it is done by a large tzibbur and has been done for a long time.
If a tzibur have been doing something for a long time, It would be very
difficult to make them change.
* R. Shmuel di Medina