Here's an excerpt from the liner notes to the LP.
Éclat was composed while Boulez was travelling in 1964. Composed but not completed, for Boulez has said that he intends to add to it [eventually, he did--it is now part of a work entitled Éclat/Multiples]. . . .
The definition of éclat is given in Cassel's New French Dictionary as "burst, sudden bursting: crash, clap, peal, sudden uproar; shiver . . ." and continues with a long list of meanings that includes "brightness, glare, glitter" and a great many other things, from which I cannot resist quoting "un éclat de pierre," which is translated as "a fragment of stone." As we have seen, the "Éclat de Pierre Boulez" is indeed a fragment: and "stone," in the special sense of "precious stone," applies no less aptly to its magical, sensuous succession of jewel-like, jingling, sparkling, flashing sounds. . . .
The work is scored for fifteen performers--piano, celesta, harp, glockenspiel, vibraphone, mandolin, guitar, cimbalom, tubular bells, flute in G, cor anglais, trumpet, trombone, viola, cello. . . .
It starts with a piano cadenza, written out, and ends with another fully written out, jerkily rhythmical concerted quick section, in effect a kind of terse "grand finale," if the term is not too incongruous, in which the six wind and bowed instruments re-enter for the first time since the single pppp chord with which they fill the silence between the pianist's first two entries in the opening cadenza.
The whole middle part, by far the longest, is a kind of free fantasy for the remaining instruments (including the piano), and is to some extent improvisatory. In some parts the note-heads are given, but no duration, and alternative dynamics are indicated. The players follow the conductor's instructions as to which of several possible readings to choose or extemporize on arriving at any given point. Boulez has indeed described the work as in this sense "a conductor's concerto, because the musicians are used like the keys of an instrument."
Another version of the same piece, conducted by the composer, can be found below, but it should really stop after 9:20; the last half-minute of this "video" is the beginning of Multiples.