Also, just to clarify one thing I found mildly confusing the first time I used it...to simply reverse video, check the "Reverse" box but leave the speed at 100%.
Checking "Reverse" and changing the percentage will make it play backwards, but at slower- or faster-than-normal speed.
Maybe I'm just easily confused. But when I saw the Time Stretch tool the first time, I kept trying to figure out the correlation between the "Reverse" and the percentage (as in: should the percentage be set to MINUS 100% if I choose reverse?)... turns out there is none.
In many NLE's and in other Time Remapping Effects, the reverse IS set by a minus percent, so you would have been correct to assume that this might also be the case here. I've not used the Time Stretch Reverse in PE, but would have pondered the same question, as you did, based on previous experiences. Adobe took the guesswork out of it with the Reverse checkbox - nice touch.