Numbers 6:24-27 “The Lord bless you and keep you; the Lord make His face shine upon you, and be gracious to you; the Lord lift up His countenance upon you, and give you peace.” So they shall put My name on the children of Israel, and I will bless them.
It was God’s command that the priest should bless the people of Israel with the above blessing. It literally placed God’s name upon them and brought them blessing. This blessing was spoken every morning following the morning sacrifice.
It is said that this blessing was not a blessing by the priest. But, because God gave it to the priest to speak word for word that it was the blessing of God Himself.
The practice today is to speak this blessing as a closing blessing at the close of the synagogue.
The understanding was that no one should look at the face of the priest while he quoted the blessing. Instead, the people would look to the ground as not to be distracted, allow more focus on the words of the blessing. Tradition says that when the temple was standing that the glory of the Lord would shine from the fingers of the priest.
I am not sure where the practice began, but while quoting the blessing the priest would hold his hands in what we know today the valkin greeting. It formed the shape of the Hebrew letter, Shin, which stood for Shaddai – All Sufficient.
From Genesis 49:25, Jacob’s blessing, Shaddai also means breast or the one who is sufficient to nurture.