What is the purpose of s Singer sewing machine needle plate 21313?

Question:This needle plate fits into my 401 machine, but it has a movable arm on the underside. What is the special purpose of this plate?

Answers:
I don't know whether you have all of the accessories of your machine. The needle plate keeps the fabric from being drawn down into the bobbin area and usually has only a large enough opening for your use (a zig-zag) opening is larger than one that had only been made for a straight stitch machine; it also accommodates the setting for a right or left zipper foot accessory. You should see if any other "feet" require the movement of that arm. I hope that helps.