Stretch
This feature is used when you know you might want to lower the tempo of the loop in your sequencer.
How Stretch works
When you slice up a loop and play it back from your sequencer, each sample will play in succession. At the original tempo, one slice will end exactly where another starts.
As you understand, when you then lower the tempo, there will be small gaps between the slices, which disrupts the flow of the audio.
Stretch is used to add an extra tail of audio to each slice, to lengthen it. This is derived from the natural decay of the sound. This extra tail of sound then fills out the gap between the slices when the tempo is lowered.
Please note that the attack of the sound is not affected in any way.
The Amount of Stretch
The Stretch knob is used to set the amount of stretch as a percentage value.
The percentage values tell you how much longer the entire sample will be after stretching. If you select the largest value (100%), for example, the slice will become twice its original length.
To decide how much stretch you need, lower the Preview tempo by the same amount as you intend to use, and adjust the Stretch parameter as necessary.
If there is a discernible gap of silence between slices you should raise the Stretch setting.
Note that Stretch is not Time Stretch! The Stretch feature doesn't lengthen the whole slice, it only adds a portion of sound to the end of it.
Stretch and the old ReCycle Formats
You should be aware of the fact that REX (as opposed to REX2) files can contain certain settings that are irreversible. For example, if stretch was applied when the file was saved, the stretching is part of the actual REX file and cannot be removed. It is possible, however, to apply more stretch, which may or may not be what you want.
For REX files it is therefore advisable to try lowering the tempo in Preview mode first, to see if you actually need to apply Stretch.
REX2 files however, only contains the actual audio file and the slices, the rest (effects, stretch setting etc.) is added when the file is loaded and can be changed at any time.