You sew the "poofy batting" to the parts you want to be poofy, with the washaway thread. Then you trim off the rest that you dont want to be poofy.
Then you lay the top (with poofy spots) on the regular batting and backing, sew neatly around the poofy spots, the washaway thread washes away, leaving the extra batting on the poofed parts, and held in place with the regular thread... viola ! see the book(s) *Trapunto by Machine* by Hari something....