"When men let grow their hair in Israel, when the people offered themselves willingly" (Judges 5:2). Rabbi Joshua ben Levi said: Whoever answers "Amen, may His great Name be blessed" with all his strength, they tear up his harsh decree, as it says, "When men let grow their hair [bifroa pera'ot]" — what is the reason? "When men let grow their hair" [read as: when retribution is loosed and undone] — because "they blessed the LORD" (Judges 5:2). Rabbi Chiyya bar Abba said in the name of Rabbi Yochanan: Even if there is in him a trace of idolatry, they pardon him. It is written here "bifroa pera'ot" [when men let grow their hair], and it is written there "for it was uncovered" [ki farua hu] (Exodus 32:25) — [by this verbal link, the loosening of hair recalls the loosened restraint of the golden calf, teaching that even such sin is forgiven].
Answering Amen With All Your Strength Tears Up a Harsh Decree
Yalkut Shimoni on Nach 46:2
בפרוע פרעות בישראל בהתנדב עם. א"ר יהושע בן לוי כל העונה אמן יהא שמיה רבא בכל כחו קורעין לו גזר דינו, בפרוע פרעות מה טעם בפרוע פרעות משום דברכו ה'. אמר ר' חייא בר אבא א"ר יוחנן אפילו יש בו שמץ ע"ז מוחלים לו, כתיב הכא בפרוע פרעות וכתיב התם כי פרוע הוא: