Apple Final Cut Pro X (10.1.2) - Manual (Page 178)

Chapter 8 Edit audio 178
Note: Make sure to adjust the audio overlap to the length you want.
3 Drag the fade handles of each clip to the points where you want the fade to begin and end.
Crossfade Fade handle
To change the fade shape for each clip, follow the instructions immediately below.
Change the fade shape when using fade handles
m Control-click a fade handle, and choose a fade option from the shortcut menu:
67% resize factor