When creating ambiances for ADR, or enhancing a vocal for a music track, real-time DSP is the way to go. But there are occasions when non-real-time, or file-based DSP is a better choice. I will give you an example.
Let’s say you need to create an ending for a piece of music. You find a hit in the music that could be an ending, but there is no ring-out, because the musicians kept on playing, after the next beat. You need to add an ambient ring out to the beat you have selected. Here’s how:
Zoom in to locate the sellected beat, trim it at its end, which is also the beginning of the next beat (use Tab to Transients). Then park your cursor at the start of the last beat (again with Tab to Transients), and press B to separate it. Control-Option drag it to another stereo track to make a copy of it at the same position on the timeline (that’s Control Alt on a Windows keyboard). Select the copy (click once with the Move tool, or twice with the Selector tool) and then Shift click with the Selector tool two or three seconds after the clip, in the blank space on the timeline. You need to do this to give the reverb time to ring out, in the new clip. That’s the point, right?
Then, with the copied clip and the blank space after it highlighted, select a reverb from AudioSuite, adjust it, and render. You now have a clip with the ring out that you need. You may need to add a fade in to the reverb clip, and maybe a quick fade out on the last beat of the music. It is important that the music does not sound like it continues, or the effect is ruined.
The point is: if you are going to add non-real-time reverb or delay to a clip, you will need to allow time in the selection for your new clip for the effect to ring out.