What is the ruling on moving the finger during rukūʿ? And does it fall under innovation (bidʿah)?

Moving the finger in Ruku' (bowing) as an act of worship is not legislated and is an innovation (bid'ah). However, moving it for another purpose, such as counting the Rak'ahs (units of prayer) or for a need, is permissible. It is similar to moving it during Qiyam (standing) and Sujood (prostration). Moving it without a need is an disliked idle act.