Tweener Documentation
AS2 and AS3.
... transitionParams:value, ...
value:Object — Additional parameters to be passed to the transition equation. Some transition equations, specially custom transitions, can optionally receive additional parameters that change aspects of the transition equation; they're defined on this parameter.
// Normal easeOutBack transition in one second
Tweener.addTween(myMovieClip, {_x:200, time:1, transition:"easeOutBack"});
// easeOutBack transition in one second, but with a much bigger overshoot
Tweener.addTween(myMovieClip, {_x:200, time:1, transition:"easeOutBack", transitionParams:{overshoot:10}});