2 things come to mind, first is the stretching I push my teeth out and pull my cheeks back sometimes. This is done with my jaw muscles. Sometimes i will sort of do a bit of that and pull my cheek diagonally up my face. I also shave accross the point of my chin and diagonally down from the outsides inwards and then just a short north south down the flat.
The second thing is angle, due to all of the small angles that a chin is made of Holding consistent angles Is hard to acheive