Wikipedia
Kindleinwiegen (Rocking the Christchild) is a Christian Nativity tradition which can be traced back to the Liturgical dramas of the later Medieval period. It is a tradition primarily from the German speaking parts of central Europe which appeared across the region in a variety of forms, chiefly though not exclusively in monasteries and churches. It was at its most widespread between the fifteenth and nineteenth centuries, and survived more strongly in Roman Catholic regions than in those areas that switched to protestantism, following the interventions of Martin Luther and others.
German language terms used for the celebration include Christkindlwiegen: the verb included in that compound word, "wiegen", indicates both rocking/cradling and weighing/getting the measure of the baby.
The celebration involved a form of congregational dancing accompanied by singing around a picture of Jesus placed on a crib or adjacent altar. The performance was led by the priests, and the singing, at least initially, appears to have consisted of duet sung by two priests.