As you can see on the picture, there's a narrow "thingy" in front of the heal on the different add ons, that is to be slid into place into the sole of the front piece.
This is obviously good for the environment, since less material is used in making this shoe + the add ons, than it would creating five whole pairs of shoes. When you travel they will also take less space than five pairs would - cause we all know we need several varieties of shoes when we travel! Another good thing is that high heeled shoes make your feet ache. With these you can simply use the high heeled part when mostly needed, and then switch to the flat ballerina heel when your feet starts protesting - they wont take much space in your bag. I really like the way they look too! I wouldn't mind trying them out.
The only thing I'm a bit worried about is how long they will hold together. It feels like the more you use them the softer they're gonna get, and the easier the back part will fall off the front part. It's a good idea. But does it work in reality?
There's also a little promo video about the shoes. You can watch it here.