9 his sister-in-law is to pull his sandal off his foot, spit in his face, and say, "This is what happens to the man who refuses to build up the family of his brother

Matthew Henry's Commentary on Deuteronomy 25:9

Commentary on Deuteronomy 25:5-12

(Read Deuteronomy 25:5-12)

The custom here regulated seems to have been in the Jewish law in order to keep inheritances distinct; now it is unlawful.