What is the difference between these two tricks?
A thumb grind uses the thumb, and it’s often done using outside of the yoyo, so it’s often used for an IRG. The thumb can be used for many different grinds.
An IRG is an inner ring grind. This is a grind in a somewhat specific region of the yoyo, often the outside area of the rim. There are often little “lips” or raised ridges to hook your finger or nail. While it can be done with the thumb, it can also be done with fingers. The Dark Magic II has a ring that is accessible with the cap on, but is easier with the cap off. With the cap off, another IRG ring becomes accessible where the mpalstic meets the metal.