Apple Motion 2 Tutorials - Manual (Page 171)

Tutorial 4 Working With Particles 171
Now, drag the spread control toward the leftmost opacity tag until the Location value
is approximately 15 percent.
Note: You can also use the Location parameter to change the Location value.
This ensures a quick fade-in for the particles.
5 Click the opacity tag near the center of the Gradient Editor.
6 Set the Opacity parameter to 100 percent.
7 Set the Location parameter to 45 percent.
The particles fade out more slowly.
8 Click the spread control between the last two tags of the opacity gradient.
9 Set Location to 91 percent.
10 Play the project.