Significa exactamente eso: "especialmente en el uso diestro de principales canonicos." Si necesitas ayuda traducciendo el paragrafo entero, dice: (excusa mi falta de tildes y 'ñ'es) "La estructura de la cancion parece a esas creadas por maestros del siglo 19 como Johannes Brahms y Robert Schumann, especialmente en el uso diestro de principales canonicos (yo no se que es eso) (en quales Brahms sobresalia) y en el uso del pulsante estilo cordal accompaniment (no se que significa esto, pero parece que es un termo musical acerca una parte que acompana a un instrumento o algo), como se puede encontrar en Schmann's "Ich grolle nicht" (de Dichterliebe, 1840)".

Oh Andrew, thank you very much for all your time, it was very helpful. I was thinking, like you, that in "canonic principals", principals could mean "principios" o "reglas", I didn't use this translation because I had in mind that "principios" would be like "principles", but maybe in English you could use "principals" like "principios" too (correct me if I am wrong). And respecto to "canonic", in Music, the "canon" is a musical form, is the same word for Spanish as for English, so, "canonic" refers to the canon. Then "canonic principals" is like "principios canónicos". For the rest of the phrase, should I interpret "dexterous use" like "uso de gran habilidad en ambas manos", instead of "uso diestro"?.

That is pretty complicated, but I will try to unpack it a little bit and maybe it can help you. Eso es un poco complicado, pero trataré mostrarte, más o menos, lo que significa o lo que pienso que significa y quizá puede ayudarte. Dexterity is like skill with hands, although it can be used in different contexts. It normally is synonymous with skill, artfulness, or cleverness - like a surgeon who has dexterity in their hands when skillfully operating on a patient, or like an archer who shoots an arrow. The composer of the song's structure demonstrated great dexterity (skillful precision and accuracy with musical principals - or using canonic principals) like a surgeon would when operating on their patient or when an olympic archer would when firing an arrow. A 'canonic principal' could mean a few things, but as I am not a student of music, I could be wrong. My initial interpretation of a canonic principal is that it is a principal (principal, meaning a 'rule', or 'law' of music composition that has developed over time) that is canonical (i.e., sacred, or to be relied upon). For example, the bible is said to be a canonical work and is referred back upon as it sets the 'precedent', 'benchmark' or 'common standard'. That said, a musical canonic principal could be almost like a sacred law of music that one uses when composing a piece of music (laws, or 'principals', such as 'strict imitation', 'development', or 'species counterpoint'). The dexterous use of canonic principals is the same, then, as saying 'the skillful use of musical laws/rules'. I could be wrong but that is what makes the most logical sense to me. I hope that it helps you!