I too have the regular SITD as well as the one for the Janome AccuFeed/
They are well worth the $, and I love the job they do.
The foot has a little "plough" on it, that sits into the "ditch" and then you just sew along.
Once I have the needle position set so the stitches "are" in the ditch, then it's easy peasy to SITD!