A similar quote about Debussy from Boulez quote has been translated as follows, on p. 217 of the following PDF of R.J.C. van Randwijck's thesis, available at http://dspace.library.uu.nl/bitstream/handle/1874/27669/rand... :

"à partir d’une extrême rigidité de conception – d’une écriture pratiquement canonique – on arrive à une souplesse de réalisation qui se laisse couramment prendre pour une improvisation flexible, d’où une succession à sens unique se trouve totalement exclue grâce à ces retours irréguliers qui changent fondamentalement la perception de la forme canonique même.

[From an extreme rigidity of conception – of a practically canonical scripture – one arrives at a suppleness of realization which lets itself be widely taken for a flexible improvisation, from which a one-way succession finds itself totally excluded thanks to these irregular returns which change fundamentally the perception of the canonical form itself.]"

(Both the French and an English translation are provided in the thesis.)

Also used in this discussion of Debussy (at http://www.ex-tempore.org/eduardo/eduardo.htm ):
"This is often achieved through embellishment using both diatonic and chromatic neighbor tones and chords, e.g., back and forth motion or trills, and can occur in compound form. Discontinuity in the form of juxtapositions, interruptions, irregular and apparently random returns, “new starts,” and other devices to different degrees also diminish directional tendencies and create a nonlinear effect."

I believe this refers to the composer returning to or repeating a theme or melodic line not in regular way, as in the symmetrical themes of a fugue (https://en.wikipedia.org/wiki/Fugue ), but in a more unexpected or irregular way.

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Note added at 1 hr (2015-09-24 15:39:14 GMT)
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Please ignore the second occurrence of the word "quote" in my first sentence, and sorry for the typo.