It consists of writing (or typing) pages of dialogue, and then cutting them up into small pieces and reassembling them randomly.
In the creation of music, a tape of part of what is needed (or the entire master tape) is cut up into short sections, which are then spliced back together out of order. This is arguably more random, since the artist cannot "see" what is on the tape while splicing it, whereas a writer inevitably can see the words on the page while they are both cutting and pasting.
The Beatles used the technique on the backing sounds for "Being For the Benefit of Mr. Kite" and an instrumental break in "Yellow Submarine". In the former case it was to create the effect of swirling calliope music at a circus, while the latter was done to a stock piece of tape from the EMI library, to avoid copyright issues.