Avid Xpress Pro - Media Composer: Trimming
You can trim a transition on either the outgoing side (A-side or tail), the incoming side (B-side or head), or both sides (dual-roller). Understanding what each version of Trim mode does is critical to successful trimming. You can use a single roll trim or a dual roll trim depending upon what you are trying to achieve. With a dual-roller trim you are not adding any more content to your program. You are simply either adding more and taking away from one shot or the other. This can appear to be very complex yet once you start playing with the basics of trimming you will find it to be very simple ans straight forward. If you perform and single-roller trim then you are adding footage from one clip. This can be done from either side and will lengthen your timeline. I will spend time breaking each down and how to use each one over the next few entires. Here are the ways you can enter and exit trim mode.
To enter trim mode:
1. Park on the transition you want to trim and press the Trim mode key 
on the keyboard.
2. Lasso the transitions in the timeline. Draw the lasso by clicking at a point above the top track in the Timeline and by dragging to surround the transitions. You can drag from right to left or from left to right, and you can lasso single transitions across several contiguous tracks.
To exit trim mode:
Click the Trim Mode button on the Tool palette.
1. Press the Right Arrow key or the Left Arrow key. 
2. Press Esc.
3. Click a specific location in the Timecode (TC1) track at the bottom of the Track Selector panel. The position indicator moves to that location.
Stay-tuned for more trimming tips.
