This page discusses various video editing techniques you can perform, using FCPX.
L-cuts are when the audio of the previous scene carries over into the next one, despite there being a picture change. J-cuts are when the audio of the of the next scene precedes the picture change, so it starts playing before the picture change.
In the walkthrough below I show you how to perform these cuts using FCPX.
The following Vimeo page shows you further applications for the cuts: "J-cuts and L-cuts, defined."