I had this therapy done years ago after one of my best friends committed suicide.

I could not believe how amazing it was and it helped me tremendously.
My therapist explained it like this: she said when we cry deeply or go through something and our coping mechanisms shut down, we tend to look straight ahead. This doesn't allow us to access parts of the brain that help us remember positive things, cope with trauma or move forward.

Watching the light go back and forth allowed me to remember events as that I had "forgotten about" (ie that it was not my fault that my friend took his life). When I cried, I couldn't watch the light, so she tapped my left knee, then my right and continued.

I left that appointment feeling lighter. It was meant to be 7 sessions, and i had reached a healthy state with closure before session 6.
I also saw this therapist for phobias but with different techniques.

If the therapist is good, this can be a wonderful option.
Good luck!