Deleuze is talking about Z; "He reflects that Zen is the reverse of Nez (nose) which is also a zig zag," and then he is talking about a scientific term for which he can't remember its name, it is a "sombre precursor," a phenomena that places two different potentialities into relation. [1]

(I have to quote all of this, its resonance with the eating-dog/ master-stick-beating story is intense, or even uncanny.)

"So, there is the sombre precursor and <Deleuze gestures a Z in the air > then a lightening bolt, and that's how the world was born. There is always a sombre precursor that no one sees, and then the lightening bolt that illuminates, and then there is the world. He says that's also what thought should be, and what philosophy must be, the grand Zen, but also the wisdom of the Zen. The sage is the sombre precursor and then the blow of the stick comes since the Zen master passes amongst his disciples striking them with his stick. So for Deleuze, the blow of the stick is the lightening that makes things visible …." [2]

This is a little like the way Leibniz understands Locke's notion of disquiet - that it is only through a sense of disquiet that anything happens - as in 'the world loves suffering more than any other pleasure.' Leibniz isn't so sure that disquiet indicates only irritation, discomfort or suffering. He writes "I would prefer to say [Leibniz is masquerading as Theophilus in conversation with Philathes, in turn a masquerade for John Locke. This allows Leibniz to quote Locke's Essay concerning Human Understanding (1700) wholesale and rebutt him willy nilly] that a desire in itself involves a disposition to suffering, a preparation for it, rather than suffering itself … there is disquiet even in joy, for the latter makes a man alert, active, and hopeful of further success."[3]

[1] Gilles Deleuze, Gilles Deleuze's ABC Primer, with Claire Parnet. Pierre-Andre Boutang. Director. 1996. Overview by Charles J. Stivale. May 3. 2003. Jan. 20. 2004. "'Z' as in zig-zag.": http://www.langlab.wayne.edu/CStivale/D-G/ABC3.html
[2] ibid
[3] Peter Remnant and Jonathon Bennett, ed. and trans., Leibniz: New Essays on Human Understanding (Cambridge: Cambridge U. P. 1996) Bk II "Of Ideas" Ch. xx "Modes of pleasure and pain": pp164-168