In most congregations (as far as I know), when the chazzan recites Birkat Kohanim (Priest's blessing) as part of the repetition of Shmoneh Esreh, after each of the 3 blessings, the congregation responds כן יהי רצון . What is the origin of this expression? Since it's a blessing (true, it doesn't have the format of a "standard" blessing (shem umalchut)) shouldn't we say אמן, instead, or perhaps, both?
Also, I noticed that when the Cohanim duchen, the cong. responds just אמן. Why the difference?
I am dealing with Nusach Ashkenaz in a shul outside Israel, as in Israel, they duchen every day.