This is a Darning Foot

Here she is.  It's a simple little foot.  The purpose of this foot is only to keep your fabric down near the throat plate when the needle is moving up and down.  When the presser foot is lowered it just barely touches your fabric.  It doesn't put any pressure down which allows the fabric to be moved in any direction with complete freedom.  I like to drop the feed dogs when I do free-motion quilting so the fabric can move as smoothly as possible.

This is the foot in it's lowered position.  

When I was experimenting with free-motion quilting I made up a whole mess of coasters with different designs.  I used fabric and batting from my scrap bin.  I didn't really care if the stitching came out ugly.  I figured out which stitches I liked to make.  And I ended up with something useful.  

I found that free motion quilting was a lot like making those drawings where you try to make your entire picture without picking up the pencil.  Focus on where you want your drawing to go next and then just keep moving the fabric around while the needle does it's thing.