As others have explained, the IDT is a dual feed system that snaps down into the foot to help move the top fabric in concert with the bottom fabric. It works fantastically. You can either use this feature or not. Some feet come with the clear area in the back so that you can engage the IDT while some other feet are solid and do not allow the engagement of the IDT.
I too have two Pfaffs. I LOVE the IDTs (both have it) I have a 2170 (for embroidery) and a 2054 (for piecing). I have 4D as well. I have taught both 3D and 4D but as I am not currently working could not justify the 5D or the new Creative Vision which I would dearly love to have as well. If either of my Pfaffs were to die, I would replace them with another Pfaff.