I recently heard (cannot recall from who or if I saw it somewhere) that after each Harachaman (הרחמן) after the 4th Bracha in Birchas Hamazon, one should answer Amen. Additionally, I was bentching today after Shalosh Seudos, and I noticed this concept brought down in the Bentcher I was using.
Nonetheless, I have never seen this happen in practice. And in the Artscroll Birchon there is no mention of it.
So, what's up with this? Should we really be saying it? And if so, why has it fallen out of practice?