Comprises: "do not" 勿 = wrapping 勹 of two long things ノ is what you should not do); HERE: the cross stroke is seen as wrinkles/folds occuring when packing i.e. pants (=long things) into a suitcase
Mnemonic
(When packing a suitcase:) When I wrapped the long things they got wrinkles, because I was in a hurry.
The app has many other functions, such as character selection according to difficulty, practice programs, handwriting recognition and success statistics!