Some early versions of Cakewalk software had time stretch features. I believe most of the new ones do as well and also are including Melodyne as part of the package.

The time shifting was promoted as a way to make tracks fit into a specific time frame either by stretching or compacting them without altering the pitch. They used the example of needing to fit a 27 second clip into a 30 second radio or TV commercial spot.

With any time or pitch shifter, the important part of using it is to avoid the appearance of artifacts that are audible to the listener. The further you shift from the original pitch/tempo, the more likely you are to introduce artifacts. The artifacts are more noticeable in certain instruments and not so much in others. Drums for example, tend to not show artifacts as easily.

This artifact creation aspect of time and pitch shifting is one of the reasons you see "recommended" tempo ranges on real tracks. If you decide to use a piano track with a recommended tempo of 60bpm in a song running at 160bpm, the artifacts and character will generally be off the charts so much so that in fact, the track will not fit well.

I don't know much about the real tracks we use, but I think I heard that they are all recorded several times and each time in different keys so that when you go to use one, there's a fairly good chance that one of those takes was in the key you're using or is only a half step or so away from one of the keys. That way, the artifacts are minimized from what they would be if RB had to transpose 3 whole steps on a piano part.

The less you have to move something.... either in time or pitch, the better off you are.

Just my 2 cents

You can find my music at:
www.herbhartley.com
Add nothing that adds nothing to the music.
You can make excuses or you can make progress but not both.

The magic you are looking for is in the work you are avoiding.