Both the artist's commentary and the written SFX say にぎにぎ (nigi nigi) which is the onomatopoeized version of 握る (nigiru). It has a wide range of meanings from kneading to massaging, but in this case the "grasping it using one's own hands" interpretation is essential too.
Also, it has a cute ring to it because it's commonly used to describe young children playing with others' hands.