This technique, (knit 1 in row below) is often abr. as K1b.
It is also called a brioche stitch.
(brioche is a kind of buttery bread, that is formed into a loaf (usually a small ball), has an X cut in top, and a second, smaller ball is stuck into the X.--so its like a little 2 ball snowman shape.--see link below for image.)
the brioche stitch does the same thing.
when you knit into the one below--the one above undoes!
(it doesn't 'run' or ladder, because its stopped by the knitting into the stitch below(so it only undoes 1 stitch)
this creates an elongated (a slipped stitch) flanked by the "wings" of the undone stitch.
Its a very decorative stitch..
the brioche stitch can be used in ribbing, in sead stitch, and various other patterns. (there is a web site just for this stitch
it Opens with a picture of a brioche, and then has lots of info on this technique.
The Brioche stitch is one that some just fall in love with.. it looks so different than "normal knitting" and yet, that is just what it is.. normal knitting!