This button allows you to reverse the playback of slices. When blue, the slices will be reversed. When gray, the slices play normally.
This slider allows you to trim the start point of the waveform. Moving the fader to the right moves the start time later in the sample.
If you Reverse a slice, then this parameter controls the end point of the waveform. Moving the fader to the right moves the start time away from the end of the sample.