Apple Final Cut Pro 7 - Manual (Page 730)

For example, suppose you are rippling a clip item on track V1 to make it 10 seconds
shorter. In a ripple edit, all clip items that occur to the right of the edit point move left
or right by the amount you are trimming. In this case, all clip items should move 10 seconds
to the left to fill the gap. However, clip items in A1 and A2 cannot move to the left by
10 seconds because there are other clip items on those tracks that are in the way. The
Clip Collision message appears.
Clip Collision messages are important because they indicate that Final Cut Pro is making
sure that clip items in your sequence aren’t accidentally overwritten. This is especially
important when you are performing ripple edits in a small portion of your sequence and
you can’t see how clip items later in your sequence are being affected.
730 Chapter 45 Learning About Trimming Clips