The threads themselves don't expand, its that they re-arrange themselves so the material is larger in total volume, but the voids within the material are also l

The threads themselves don't expand, its that they re-arrange themselves so the material is larger in total volume, but the voids within the material are also larger. The solid material itself, of course, must be conserved. Think of a sponge: normally if you stretch it the pores get smaller. Auxetic materials can be engineered so the pores get larger, but the amount of sponge stays the same.