If the two lines were placed next to each other (as opposed to stacked), the lines would form something conceptually similar to a palindrome.
The name 'crab' refers to the fact that crabs are known to walk backward (although they can also walk forward and sideways).
It originally referred to a kind of canon in which one line is played backward (e.g. FABACEAE played simultaneously with EAECABAF).
An example is found in J. S. Bach's The Musical Offering, which also contains a table canon ("Quaerendo invenietis"), which combines retrogression with inversion by having one player turn the music upside down.
This music theory article is a stub.