The water causes many tiny cracks to form in the outer layer of glass. The timing of this step is critical: too much time in the water and the shape will be too stiff to blow out; too little time and the crackle will disappear in the later reheating. One second too long or too short and the piece will not turn out. It takes considerable practice to judge exactly when to remove it. At just the right moment he removes it from the water. If you look very closely at the photograph you can see many very fine cracks.
