The app was called "Orphion", and it had several methods for detecting gestures/articulations: a gentle finger tip touch, a tap, and a push down. Apple cited forbidden private API use.

http://createdigitalmusic.com/2013/06/hands-off-apple-wants-...

  Apple just called me to nicely tell me I use a private 
  API function to sense the area of the screen which is 
  covered by the finger for its articulation gesture ([CDM] 
  wrote about it). This is crazy – thousands of users love 
  it for this and it makes Orphion so expressive.


  The app will be removed from the App Store in two weeks 
  if I don’t submit a new version without it – and I 
  currently can’t think of Orphion without this gesture.


  So what I can recommend is


  1. Everyone who wants to have the “original” Orphion get 
  it NOW from the app store (https://itunes.apple.com/us/app
  /orphion/id495465097), it will be only be there for a few 
  more days

  2. Backup the current version to keep it. (Michael Tyson 
  from Audiobus made this great tutorial)

  3. Tell Apple to make this great function officially 
  usable in apps (Any ideas how to do that?)

  So far… let’s see if this is really the end of Orphion.

...looks like the petition worked and Orphion was allowed to keep the gesture functionality; https://itunes.apple.com/us/app/orphion/id495465097