I’m using US terms.

what you can do with it:

This decorative stitch lies on the surface of your work.  It doesn’t look the same on the back, so you might want to save this for works with a specific right and wrong side.  You can add it to anything though — even hdcs and scs.

how to do it:

Work a few rows of whatever stitch you like (I’ve done dcs here):

Now we’re going to insert our hook in a very tricky place.  Look where the scissors are pointed:

It’s the chain that the dc above was worked into.  Insert your hook from front to back and pull up a loop:

You can see there that the scissors are pointing out the next place for our hook.  It’s where the next dc was worked into the one below.  Insert hook from front to back, pull up a loop.  Pull the loop through the first surface stitch, too:

Keep going across your row.  It’s essentially making chains on top of the dc background:

You can alternate colors for a neat look:

That’s it!  Pretty neat, huh?