It's really simple and works nicely. Right click the event on the timeline, click on 'insert/remove velocity envelope', switch your cursor to the 'envelope editing tool' (the icon for this is in the bar at the top of the screen), right click the event again and select 'show velocity envelope', you'll see a blue line running horizontally across the event on the timeline. Using your envelope editing tool cursor, pull the blue line down to slow the event action, pull it up to speed up the action. If you pull down far enough, the action will reverse.

You'll want to play around with this a little, as the contents of the event (I guess I should call it the range of the event) will change as you move that blue line.

Hope this helps.

Caruso