Reference for Yevamot 203:16
מיתיבי הותרו רצועות מנעל וסנדל או ששמט רוב הרגל חליצתה פסולה
is because it was he that slipped it off; had she, however, slipped it off, her <i>halizah</i> would have been valid; [and, furthermore this applies to] the greater part of the foot only<span class="x" onmousemove="('comment',' Lit., 'yes'. ');"><sup>37</sup></span> but not to the greater part of the heel!<span class="x" onmousemove="('comment',' How then could Rab state that permissibility to marry a stranger comes into effect as soon as the greater part of the heel had been released. ');"><sup>38</sup></span> — The 'greater part of the foot' has the same meaning as 'the greater part of the heel'; [and the reason] why he calls it 'the greater part of the foot' [is] because all the weight of the foot rests on it.