It's not that it doesn't work, because it works in most instances, so your T is right to be confident that it can help, at least to some extent. And you're also right that there's no concrete info on how it actually works. Some of the explanations sound like, well, BS. There are some scientists that claim, using evidence, that EMDR is really nothing more than good old exposure therapy, and that it is the exposure component that's responsible for improvements, not the rest. It would be kind of like if your CBT therapist also decided to dance for you and call it Dancing CBT. If improvements happen, it's most likely due to CBT, not the dance.

I'm being humorous (or attempting to) but the point is but I think it's worth a try if this is not too expensive, even if the method remains controversial in some ways.