Also known as Inner Rim Grind , Thumb Grind or IRG.
The Inner Ring Grind is a grind trick in which something (typically the 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 an adequate rim weight groove.
Inner ring grings became very popular in the early to mid 2000s, prior to the introduction of hubstacks. Many yo-yos were designed with a careful attention to weight distribution, which resulted in an "inner ring" to maximize rim weight. This integral design element became the basis for the popularity of IRGs.
Although IRG's had been experimented with earlier, they were notably popularised by Andre Boulay in his video "Dark Magic." They are often referred to as "thumb grinds" although the term "thumb grind" could also refer to when the whole yo-yo grinds sideways with the pog on top of your thumb, or when the yo-yo itself grinds on your thumb, first done by Tommy Gun in 2000 (video). The term Inner Ring Grind was coined by Tommy Gun in 2004, which he later regretted.
Performing an Inner Rim Grind
First off you want to have a yo-yo with a nice size inner edge where you can put your thumb in. You can see this simply by taking the cap off of your yo-yo with a suction cup or looking on the inside of the yo-yo. Now lets start. Ok throw a trapeze make it so its not far away from your thumb, now take your middle finger and bring it together or bring your non throwhand hand together, now stick your thumb in the gap from the yo yo, heres a tip bring your thumb with the yo yo up not to get it stuck on the string. OK you did it. But the yo yo is going to start revolving as soon as it touches your thumb.