From The 3AM Epiphany by Brian Kiteley, this exercise is called “Outrunning the Critic.” I’ve summarized it here:

• Write one hundred short sentences about a character in a piece of your fiction.
• Don’t lift your fingers from your keyboard for all one hundred sentences (then go back and revise tomorrow).
• The sentences should not connect, nor need they follow one another logically.
• Be careful not to use the name of your character or a pronoun to start each sentence; the key to this exercise is to relax and let your mind find new material and detail.