Normally when you trim regions by dragging with the mouse, it affects only the selected regions. Their lengths are directly affected by the trim operation, but nothing else is. Sometimes though, you'd like to trim a region that directly adjoins another, and keep this relationship the same - you're not trying to make one of the regions extend over the other - you'd like the junction to move in one direction or the other as part of the trim. This requires trimming both regions on either side of the junction, in opposite directions. Push/Pull trim, activated by pressing shift key before starting the drag, will do just that. Here's a few pictures to show the difference in the results of a normal trim and push/pull trim. First, the initial situation:
Now lets look at what happens after we trim the right hand (selected) region by dragging its starting position earlier:
You can see that it now overlaps the earlier region and a cross fade has been created between them.
Lets look now at what happens if we do the same trim, but using shift-drag to turn it into a push-pull trim instead:
There is no overlap, and the end of the earlier region has been moved along with the start of the later region, so that they still directly adjoin each other.