Yes, the app is good. Like you, I thought it was just because you were concentrating on something - but I don't think that's the whole story, because I've also done breathwork apps where you are concentrating on breathing in and out, I've done mindfulness and meditation, etc etc - and none of that seems to work as well as tapping has done for me in the last 48hrs.
I feel really relaxed and like my whole body has been given a big hug or something, I think it does something beyond just making you concentrate.
With the app, download it and sign up for the free 11 day trial. Then just go to your name on your phone and Subscriptions and cancel the free trial. You will still get 11 days free access and won't be charged after that. (Unless you sign up again.)