When you put Frequency to a negative value, it will play backward, for example if you put
default * !Rate
in the Frequency and choose the range of !Rate to allow for a negative number in VCS, it would allow you to change the playrate in real time and allow it to play backward.
Or if you just want the sample to be played backward in the same speed, you can use such as:
default * (!Backward of: #(1 -1))
Then !Backward will be a button for you to switch playing forward or backward.