Question

MongoDB: What is a pinned cursor? what behaviour can produce a pinned cursor?

Answer 1

A pinned cursor is an internal implementation detail where the pinned flag denotes an open cursor that is actively in use and should not be deleted. 
Cursors are generally pinned for a short period of time. Example: a find() or getMore() operation will pin a cursor to prevent it from being deleted while fetching a next batch of results, and unpin the cursor when results are returned.