Keep your left thumb in place. Reach your left index finger through the space under your left thumb.

保持左拇指不动，把左食指伸到左拇指下方的空间。

" Under my thumb" is not usually used in a positive way;in other words, the person who is under your thumb is considered weak or inferior.

“Under my thumb” 通常不以积极的方式使用；换句话说，在你的拇指之下的人被认为是软弱或低人一等的。