The radical niku is typically at the left side of a composition and drawn as: 月, which looks equal to: "moon, month 月" but still has the meaning 'body part'.
The upper stroke 一 can be seen as the horizon, with variations of sun 日 and two legs 儿, which stand for a downward motion: Below the horizon sinks the sun (at sunset) in the west.
The app has many other functions, such as character selection according to difficulty, practice programs, handwriting recognition and success statistics!