Basically what ChainFlow said. Or the one handed Kimura grip description with the head 'in' instead of 'out'.
So, for example, from closed guard, I shift my weight/angle so I'm on my right hip and I can reach over their shoulder and across their back with my left arm. Then, instead of just gripping their lat/armpit, my arm continues and gets inside control of their left wrist/forearm. Often I'll have to get a grip on their left wrist with my right hand (as you would to set up a triangle or something) and then push their arm back to feed it to my left arm.
I call it the "Khabib" because it's very similar to the grip he often gets from top of turtle (I'm fairly certain he used it a lot against Michael Johnson). Askren gets a similar grip. As ChainFlow pointed out, it's basically a common wrestling grip, except I use it from closed guard. I'm a very long, lanky dude, so it's entirely possible that it won't work for a lot of people. Or rather, it's very dependent on the size of your opponent. I struggle to get it on very broad/thick grapplers. But if I do get it, I'm usually guaranteed a triangle, guillotine, sweep, or sometimes a strange kimura/hammerlock variant. It just completely immobilizes them (unless they're much better than me, in which case they blow through it... as they do with everything I try to do haha).
You can see the grip a lot in this BJJ Scout breakdown of Khabib (like at the 30 second mark)...