As you might expect, a “J-Cut” is just the reverse of an L-Cut. That is, you let the video of the first speaker continue for a bit as the second speaker starts to talk.

The clip on the right illustrates. The first guy finishes, but we continue to see him as the second guy begins talking. Then, after a few seconds, we cut to the second guy.

Why would you do this? Partly it’s to get some variety, and partly it’s to give viewers the performance that better tells the story. Often the character not speaking is really saying more through his expression than the person who is speaking.