The effect is achieved by plunging the semi-blown hot gather of glass into cold water - the outside of the glass cracks. It is quickly removed, then reheated and reblown, to stretch out the cracks which appeared on the surface.
The Ysarts used this technique in their "Cloissonne" bits, Michael Harris used it to make his "Crizzle" design. WMF used it in their Ikora ranges. It has been a popular technique for a long time.
I've been told the official name for the process is actually "primavera".