Duncan's Swivel-Eze yo-yo finger tightness control ring with four extra cotton strings. The control ring consisted of a strip of velcro (hooks on one side; loops on the opposite side) with a brass swivel in the middle facing the hook side. The ring served two purposes: it controlled string tightness to make it more comfortable on your finger and the swivel action allowed the string to always stay at the proper tightness.
While the ring itself reduced string tightness around your finger, it introduced a different problem. The swivel attached to the velcro ring created a pronounced "bump" that was likely uncomfortable on the finger.
This product was short-lived and it is hard to find now.
Of interest... Over the years yo-yo makers and inventors have tried various methods to maintain proper string tightness and to alleviate "Yo-Yo Finger" -- the pain caused by prolonged wear of a yo-yo string around your finger. Here are a few problem-solvers that have appeared on the market since the early 1980s (including a patent from 1930):
