Otherwise known as shrimp stitch, reverse double crochet (in the UK), or reverse single crochet (in the US).
Crab stitch is a twisted version of double crochet worked left to right rather than right to left. You can work one row of dc, then without turning your work go back along the row in crab stitch, then work in dc again.
Kake has not entirely got the hang of this yet. Going across in crab stitch I can do now, thanks to the illustrations at Denise's crochet paradise, the photographs at Needlecraft University, and the video and illustrations at StitchGuide. What I can't figure out is where I'm supposed to be putting my hook when I go back across for the next dc row. I can't find any photographs or illustrations of that row being worked. Maybe it's never supposed to be worked on top of, but is only used as an edging?
See Also: Crochet Tips