Tied round the wrist
A UNESCO case study of the Baci Su Kwan says the rite centres on an offering tray of food, banana leaves and flowers prepared by members of the community, and a respected elder, usually a former monk, recites Buddhist chants and mantras. White thread, standing for peace, good health and community solidarity, is tied round the right wrists of all the participants, and the ceremony ends with a shared meal. It is held for rites of passage such as marriages, ordinations and the birth of a child, and for someone recovering from illness, an accident or hardship, to gather the community’s support and bring the person back into community life. Wikipedia’s article adds that the thread goes on the person being wished well and on the wrists of all the guests who gather for them.
To verify: Who ties the strings: our sources say everyone taking part has one tied on, but not exactly who ties them