The possibility exists that a brother-in-law and sister-in-law might not exactly be enamored of one another, with the result that the idea of yibum might be distasteful There was, however, an alternative: a shoe-removal ceremony called "chalitzah" (literally, "the shoe-removal ceremony").
In the chalitzah ceremony, the yevamah (the deceased's widow) removes a special shoe from the yavam. She then spits in his direction and says, "Such shall be done to the one who refuses to build up his brother's house." After this, the yevamah is given a document to the effect that the chalitzah ceremony was performed so that she is free to marry.
The reason for this mitzvah is to demonstrate that it would be fitting for the brother to perform yibum with the widow. However, he declines, turning the yevamah from a potential life partner to a virtual stranger. Her actions in the chalitzah ceremony symbolically represent this transition.
This mitzvah applies in all times and places. It is discussed in the Talmud throughout tractate Yevamos. It is codified in the Mishneh Torah in the fourth chapter of Hilchos Yibum v’Chalitzah. This mitzvah is #217 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos and #46 of the 77 positive mitzvos that can be observed today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.