I've just given it a try, but I'm not sure it worked properly for me. ...
I have to admit that I have now repeated the hearing test five times, as I learned how to get the most ear-acceptable results a little more each time. The final time, I kept an eye on the curve I was creating, and perhaps was less honest about the point at which I could no longer hear each tone - I tended to more the slider a bit to the left of the point at which my ears told me the tone was inaudible. Otherwise the created curve was rather drastic-looking. And I left the extreme high frequencies unboosted, as with my ears that's a lost cause and could even damage my headphones! Lastly, I save three versions of each test, using the drop down box which allows you choose full, weak, weaker - and then when applying the eq, I find the "weakest" version tends to be less of a shock to my ears.
What it has revealed is that a recording from the past which I recently uploaded to YouTube having applied some EQ to the high frequencies should have had its EQ left alone, now that I have replayed it with my hearing-adjusted headphones! Oh well...
The creator of the app has made a slew of videos about its general use on YouTube. Worth checking out to grasp what else it can do.