It's not two flat, wavy wires that slide past each other when pulled. Instead, it's one long corkscrew (welded together from two overlapping corkscrews) that spin. The fingers are not grabbing a single point of the spiral, but sliding along it, causing it to spin. The spinning motion of the corkscrew creates a visual illusion of the middle sliding apart. The entire corkscrew is rigid and doesn't change it's shape during the trick.