This radical can be seen as a person 人 who has the right leg horizontal 一. He is thus lying. (Further radicals with "person" are: sitting 匕, sitting cross-legged 夂, kneeling ⺈, skilled 才, bending マ,卩, slumped 㔾 or husband 夫)
The app has many other functions, such as character selection according to difficulty, practice programs, handwriting recognition and success statistics!