Yevamos 101b — Can an Onen Perform Chalitzah? The Chacham Tzvi’s Powerful Question

If an onen is exempt from mitzvos, can he perform the mitzvah of chalitzah? The Chacham Tzvi asks a stunning question: at the very moment every yibum begins, the brother and widow are both onenim—so why doesn’t that invalidate every case of yibum from the start? We explore the Chacham Tzvi’s question, the Yaavetz’s answer from Tosafos, and the Noda BiYehuda’s fascinating principle about a temporary exemption causing a permanent loss.