I've never done curves PP. I always cut my two pieces with a template, notch or mark my center point and 1/2 points lay my concave piece ontop of my convex piece, right sides together, pin on my marks and ease the convex piece in with more pins.
http://www.quiltingboard.com/tutoria...*-t132011.html
Here is a tute on the quilting board but she does it convex on top.
I think it is a preference thing. I don't understand how it can be done via PP.