One method to process a digital signal is to rearrange the order of the samples contained in the signal. More specifically, to put the samples in the opposite, or backwards, order.
This creates a signal which has been reversed over time. The last sample becomes the first sample. The second-to-last sample becomes the second sample. This continues until the original first sample is placed as the last sample.
This effect produces some interesting results. One famous example can be heard in the guitar solos of Castles Made of Sand by Jimi Hendrix. This effect was originally created by feeding analog tape is the reverse order through a tape machine.
With digital signals, this effect can be created by indexing an array to extract the elements in a reverse order.
Here is a link to an example about creating different types of arrays used for indexing samples in a reverse order.