Please make it clear whether you've read these phrases in print, or heard them spoken aloud. "Pandia-tonic clusters" sounds to me like "pentatonic clusters." The pentatonic scale is the primitive five-tone scale that can be played on the black keys of a piano. A "cluster" is several notes played together. So a "pentatonic cluster" would be the various kind of tone combinations you would get from pressing several black keys at the same time. "Flat-submediant key switch" I'm not at all sure about, but I don't know much about music theory. Wikipedia tells me that the "submediant" is the sixth note of the ordinary diatonic scale. In simple music, we are used to hearing chord progressions and key changes that move around the circle of fifths, and this sounds like some MUCH more sophisticated progression.

It sounds like advanced music theory terminology. Unfortunately, I only took one semester of music theory. This website might help, although I couldn't find "pandia-tonic." http://dictionary.onmusic.org/search?utf8=%E2%9C%93&term_search%5Bquery%5D=submediant&commit=Search

Perhaps you are reading English that is above your level?

These phrases entered the English mainstream in 1963 through a famous article by William Mann describing the music theory used by the Beatles. Nearly every popular song from the 1950s to today uses the same simple and specific rules to generate a hit, but William Mann pointed out how the Beatles were breaking these rules to create interesting compositions. This lead to a greater respect for the band as previously few adults took the music of the Beatles seriously. Pan-diatonic clusters is a fancy way of saying that no notes appear anywhere in a song that are not native to the home key, and that they are all considered consonant amongst each other. A flat-submediant key-switch refers to the use of a non-diatonic chord, in this case a flattened sixth. In the song "I Saw Her Standing There", when the lyrics go "She wouldn't dance with another - OOOOOOHHH!", the "OOOOOOHHH!" bit is sung over the flat-submediant key-switch. Don't worry if you don't understand, most English speakers don't either. :) The full article is brilliant though, and if you haven't read it you can find it here: http://aeoliancadence.co.uk/ac/Most_famous_article_in_pop_music_history__William_Mann_Aeolian_cadence_pop_music_appreciation.html