I am not a tattoo expert, I just happen to be aware of this optical illusion and the movement looked similar, example here: https://www.reddit.com/r/explainlikeimfive/comments/nstvwu/eli5_how_does_this_optical_illusion_work/
Basically if you have contrasting colors against each other they're processed at different speeds in your brain. His skin is lighter than the tattoo's relatively dark color/outline and I think it's enough to cause the illusion because that area of his pec is shaved and the rest isn't.
And it probably wouldn't happen in real life because up close you'd see enough detail/imperfection in his skin to break the illusion.
It's a thinker.


