It's done by poking wet sticks in. At least, that's how the unsophisticated ones are done. I don't know how they stop the steam from creating big blow-outs (as in the Scandi orchid vases, Webb's Flair or even Sam Herman's blown out tops) but I'd think something needs to stop the hot glass sticking to a stick or needle shaped tool.
Your Sarpanevas may have required something a lot more fiddly and difficult. I am actually struggling to see how the Sarpaneva's could possibly have been made, they are so clear, delicate, sophisticated and elegant.
I suspect my modern one is from India.