The Gray Rock technique is a useful strategy intended to deflect any attempts on the part of the narcissist to bait or trigger you. It involves being as dull and uninteresting as... a gray rock!
Narcissists are all about gaining attention, control, dominance, and power -- and they'll use every trick in the book to satisfy these needs. They seek to engage you to either lure you back in—or to prove that they can still get a rise out of you, i.e., they still have "power" over you. (Like a toddler, to a narcissist, even bad attention is desirable.)
Ideally, when dealing with a narcissist, you're able to go No Contact. Unfortunately, this isn't always possible, for instance, in co-parenting or workplace situations, or if, say, the narcissist is a family member. In these situations, the Gray Rock technique is a good one to have in your toolbox.
This technique involves a "Just the facts, Ma'am" approach by sharing only necessary information. By showing little interest, and no emotional reaction, being as brief as possible, and disengaging from them as soon as is feasible, you take some of the wind out of the narcissist's manipulative sails. Who wants to engage with a boring old gray rock?