From the help file, under Event Envelope:
Right-click a video event, choose Insert/Remove Envelope from the shortcut menu, and then choose Velocity from the submenu. A velocity envelope is drawn on the event.
Drag the envelope up to increase the playback speed of the event, or drag down to decrease its speed.
Setting a velocity envelope to a negative value will cause the event to play in reverse. Video events can only be reversed if there is media to reverse: an event cannot be reversed past the beginning of a media file. An event that has been reversed will play backwards until it gets to the first frame and will then hold that frame for the duration of the event.
Steve Mann
MannMade Digital Video
http://www.mmdv.com