If by "understanding" we claim to be able to know and understand why Movement Y--its specific music--follows inexorably Movement X, put me down as a skeptic. I am not talking about the well-understood nature of most genres: for symphonies, fast, slow, schezo-ish, fast, etc. There are certainly many works where themes, melodies are recapitulated in a following movement, or where, as in some program music (Nightride and Sunrise, Pastorale), an idea predictably follows another. Otherwise, I am of the opinion that, for many works, one could use a random-number program to select any given, say, scherzo for any other in the works of any given composer. This is in conflict with a notion that, A) once a composer sets down the first few notes, the rest of the work follows inexorably, or B) unless the composer, as it is sometimes said of Mozart, finds it formed as a complete unit in the composer's mind, with the same conclusion. I will concede B but balk at A. This is different from the composer choosing to generally express certain emotions/impressions in a symphony because an extra-musical message is to be carried by the music--Shostakovich's 5th symphony, or Tchaikovsky's 4th spring to mind.