Constant and Variable Timewarps
Timewarp playback rates can be either constant or variable. A constant playback rate can be used, for instance, to play a clip in slow motion. A variable rate can be used to create an effect where a clip plays at its normal rate until a given action occurs, at which point the frame rate increases drastically until the action is finished, then returns to normal for the duration of the clip.
When you create a timewarp, head and tail handles are automatically created. The number of head and tail frames is determined by the speed and duration of the source. On variable speed timewarps, heads and tails are infinite (source frames are reused infinitely, as determined by the timing curve).
Because constant timewarps have a continuous speed, the curve for the timing channel in the Timewarp Editor is linear, containing one keyframe.
If you add keyframes and change the linear curve to hermite or natural, the timewarp converts to a variable timewarp. To edit the speed and curve of a variable timewarp, you can manipulate the timewarp channels and use trimming operations. A variable timewarp is identified in the timeline by a “*” in place of the element's head and tail information.