What is the mechanism of collage?

Max Ernst (1891-1976) - Listonosz Cheval, collage 1932.
"I am tempted to see in collage the exploitation of the chance meeting of two distant realities on an unfamiliar plan [this being quoted as a paraphrase and generalization of Lautreamont's famous phrase: "Beautiful, like the chance meeting of a sewing machine and an umbrella on a dissecting table"] or, to use a shorter term, the culture of systematic displacement and its effects...A ready-made fixed, once and for all (a canoe) , finding itself suddenly in the presence of another and hardly less absurd reality (a vacuum cleaner), in a place where both of them must feel displaced (a forest), will by this very fact, escape to its naive destination and to its identity: it will pass from its false absolute, through a series of relative values, into a new absolute value, true and poetic, canoe and vacuum cleaner will make love. The mechanism of collage, it seems to me, is revealed by this very simple example. The complete transmutation, followed by a pure act, as that of love, will make itself known naturally every time the conditions are rendered favourable by the given facts: the coupling of two realities, irreconcilable in appearance, upon a plane which apparently does not suit them."
Max Ernst (1936)