(Devarim 25:9) "Then his yevamah shall draw near to him before the eyes of the elders": We are hereby taught that she goes after him to the elders. "before the eyes of the elders … and she shall spit": spittle that is visible to the eyes of the elders.
The Ritual of Yibum and the Reluctant Brother
Sifrei Devarim 291:1