釋氏要覽 2.21

2.21

謂頭向下。虛搖而不至地。即周禮第二拜也（凡釋氏致書俗人。即不得言稽首頓首。謂涉拜也）。

It means the head nods downward, swinging in the air without touching the ground. This is the second bow in the Zhouli. (Whenever Buddhists send letters to secular people, they must not write qishou or dunshou, as these imply bowing.)