Apple Final Cut Pro 7 - Manual (Page 167)

Chapter 8 Adding Video Filters and Transitions 167
You can lengthen or shorten a transition by dragging its edges. But keep in mind that
changing the length of the transition also changes what content appears just before or
after the transition. For example, increasing the duration of a transition might clip vital
footage that you want seen before the transition.
The last full frame to be seen
is now earlier in the clip.