Apple Final Cut Pro 5 - Manual (Page 1109)

322 Part II Project Interchange
If you make a clip play backward by selecting the Reverse checkbox in the Speed
dialog, the speed indicators turn red to show that playback is going backward in time.
When you make variable speed changes, the duration of the affected clip stays the
same. The Timeline speed indicators display all speed changes—fast forward, slow
motion, and reverse—happening within the clip. In particular, since variable speed
changes can also transition smoothly from one speed to another, the spacing of the
speed indicators indicates the acceleration of the speed change.
For example, in a clip with a simple variable speed change accelerating from slow
motion to fast forward over the full duration of the clip, the speed indicators start off
farther apart from one another, and then gradually move closer together as the speed
of the clip accelerates faster and faster, until the end of the clip is reached.
Reverse indicators
are red.
Variable indicators