Dart Manipulation is a means of changing the front (and sometimes the back) bodice fullness controlled by darts.
Darts are used to shape the fabric over the body. If there were no darts at all the garment would be very loose and flowing like a tent dress. Or it could be almost column-like in the case of a chemise dress. But even in a chemise and a tent dress the shape is affected by darts that were there in the basic pattern. The reason why they are no longer visible is because the darts existing in the basic pattern have been manipulated causing a change in the shape of the pattern.
The following dart manipulations and pattern transformations come from a handout I got back in the late 1970s from a friend who was going to Traphagen School of Design.* I’ve no idea where this material came from and I have never tried any of these transformations. I do not know how they would make up in a muslin but for illustrative purposes I think they are good to give you an idea of how dart manipulation works.
I don’t have a year for the source material that follows but the styles look very 1940s-1950s.
*The Traphagen School of Design is no longer open.