Inner Ring Grind

From YoYo Wiki

Also known as inner rim grind and thumb grind.

The Inner Ring Grind is a grind trick in which something (typically your thumb) is placed inside the ring of one half of a yo-yo while spinning, thus grinding on the object. These are not possible with all yo-yos due to sidecaps or lack of a groove.

These tricks were popularised by Andre Boulay in his video "Dark Magic" although they had been done before. They are often referred to as "thumb grinds" although although the term "thumb grind" could also refer to when the whole yo-yo grinds sideways with the pog on top of your thumb.