The defining characteristic of these etudes is Ligeti’s obsession with complex rhythms and what he called "illusory continuity." While the score often looks like a wall of black notes to the performer, the listener hears shimmering textures that seem to evolve organically.
If you are downloading the PDF to sight-read, stop right there. You cannot sight-read Ligeti. Here is what you are getting into: ligeti etude pdf