Musar על ויקרא 5:8

לא יבדיל . Concerning the law not to sever the head of a bird serving as a sin offering (5,8). Rabbi Menachem HaBavli reasons that seeing the sinner had been קשה עורף, stiff-necked vis-a-vis G–d when he committed the sin he now wishes to atone for, this is symbolized by the Torah specifying that he must "pinch the head of this bird at its neck without severing it."