Corrugated Ribbing

Corrugated ribbing is worked such that the main color is knit and the contrast color is purled. It is not as stretchy as regular ribbing. Do not tug on the floats created on the wrong side of the work; they should be loose rather than tight.

You can work holding both strands in either hand or one strand in each hand; just be consistent in which yarn is on top versus below.

Knit Picks has a couple pictures here: (scroll down).

Also check out these Knit Picks tutorials about stranded knitting: