Positive commandment · Mitzvat Aseh #217 of 613
To remove the yavam's shoe [if he does not marry his brother's widow]
לחלוץ ליבם
The verse · Devarim 25:9
וְנִגְּשָׁה יְבִמְתּוֹ אֵלָיו לְעֵינֵי הַזְּקֵנִים וְחָלְצָה נַעֲלוֹ מֵעַל רַגְלוֹ וְיָרְקָה בְּפָנָיו וְעָנְתָה וְאָמְרָה כָּכָה יֵעָשֶׂה לָאִישׁ אֲשֶׁר לֹא־יִבְנֶה אֶת־בֵּית אָחִיו
“then shall his brother’s wife approach him in the presence of the elders, and loose his shoe from off his foot, and spit in his face, and shall answer and say, Thus shall it be done to that man that will not build up his brother’s house.”
The Rambam’s words
To remove the yavam's shoe [if he does not marry his brother's widow], as [Deuteronomy 25:9] states: "And she shall remove his shoe from his foot."
לחלוץ ליבם, שנאמר: "וחלצה נעלו מעל רגלו" (דברים כה,ט).
Why · Sefer HaChinuch #599
Release (chalitzah)
“It is from the roots of the commandment [that it is] since this woman is fitting to serve this man instead of his brother from the reason that we gave above regrading the commandment of levirate marriage and [yet] he does not want her.”
Who & when
“And it is practiced in every place and at all times by males.”
Where in the Mishneh Torah
Hilchot Levirate Marriage and Release הלכות יבום וחליצה