gateing-ducking help plz

[mehrads]

Hi all,

I'm a protools LE 8 user.
I have two tracks in a project both routing to the main output.I tried to use the expander/gate Dyn 3 on one of them (the target) and feed it's side chain input whith the incoming signal of the other track (source) in order to Duck it.
but whenever a signal comes into the sidechain,I here my traget track plays simultanesly with the source one..what I need is to revere the process of gating here..how can I achieve it with protools plugins ?

hope i could explain my aim well.

[los marbles]

Create a send on the trigger track (for example send to bus 10), click on the "pre" on the bus fader, and then drop the expander/gate, with the key input set to bus 10, on the track you want it to affect.