At the end of the line bring the needle back up slightly to the side of the final stitch in the line of chain stitches. Then run the needle under each segment of the chain stitching.

Vary the number of strands to get thicker or thinner lines.

Uses
This stitch is great as stems for flowers and vines.

This stitch works for lettering too:

This stitch makes for an interesting fill-in stitch:

Both examples above used variegated threads for the whipped chain stitch fill-in, which helps add interest. As you can see this stitch provides an great texture as well.
Experiment a little and see what else can be done with this stitch!