Hello everybody!
Wonderful site!
I'm brand new to knitting and this has helped so much. I am knitting a practice scarf, using sections of garter stitch and small sections of ribbing (k1p1). My question is: why does the yarn pull in at the ribbing sections? It doesn't happen immediately when I switch to ribbing, more like after I switch back to garter stitch I notice that the sections of ribbing kind of develop an hourglass shape. Does that make sense? Can anyone explain why this is happening? Am I doing something wrong? I'm knitting the knit stitches and purling the purl ones--except on the first row, of course, after I switch from garter st, when they're all knit stitches.

Thanks for your help!

If I'm understanding you correctly, that's perfectly normal. That's what ribbing is supposed to do. It creates a more stretchy kind of fabric (think sock tops) that pulls itself in but can stretch out as needed.

Different stitch patterns create different effects. This is why the bottom of sweaters, cuffs, or the tops of socks as Mason said, are ribbed.