In case you don't know what a Dior dart is, it's a short dart from a side panel. Burda often uses them and Hot Patterns used it for their new Montpelier dress which I am making.
As you can see, this dress has a side panel which extends as the under sleeve for this dolman sleeve, if that's what you call this cut on sleeve? But, regardless It has a side panel with a dart in the front panel. The side panel is far from the bust apex. A princess seam would run over the apex or close to the apex of the bust. Here is an example of how Dior used it in the 2015 Spring collection.
I love this example with the top stitching. I turned my single Dior dart into two to make them smaller. Jennifer Lawrence had a dress on that had three small darts from a side panel that was very attractive too. It really molded the bust beautifully.
This is from The Cutting Class site. http://thecuttingclass.com/post/117112011493/bust-shaping-panel-lines-dior
My alteration of the Montpelier dress looks like this, showing how I've divided the dart into two, but I am going to make two front versions one with one dart and one with two as it is here to see which I like better. I also shortened the dress since this is a very awkward length on me, to just below the knee.
Here's the Y dart I made. Because I have a low bust I made a smaller Y section, about 1/2" with the remainder, about 1" in the side dart, because it widens the upper chest where I am not wide. The sleeve gets cut off in this type of sleeve so that you can make the second part of the fba like you would in a regular armscye. Then it gets re attached and any opening filled in with paper. This is smaller than the usual fba I make. But, when I tissue fit it this is the width I needed. It remains to be seen if this sews up well in a muslin.