--Previous Message-- : I think it's the start of the train, see last : picture.
Yes, I think you're right. The train is not visible in most of the pictures (but one can spot it also in the first and the second to last image) and the "brooch" is rather an embroidery to emphasize the attachment of the train and the asymmetry of the dress.
One can imagine what the designer wanted to achieve, but it's a very clumsy and amateurish attempt on stylish evening attire.