Classical and studio glassblowing is a two stage process. The main body of a vessel is prepared on a blowing iron and the neck and rim of the piece is finished on a punty iron. Water is sometimes used to chill the neck and create a weak point to enable the transfer to the punty iron. Small, relatively thin pieces do not always need these;indeed water can irreversibly crack and ruin a small piece. On thicker pieces where there is lot of heat retention and not sufficient temperature contrast between the neck and the body of the vessel, water is used as a coolant to crack the surface of the neck, so that the piece detaches when the iron is tapped, or hit with a piece of wood. Applying water to hot glass is not normally a good idea as you can crack the piece or get frizzle marks. So accurate application is needed. Once on the punty, the cracked rim can be melted, and sheared so any cracking from the water is lost.
I am afraid this is one of those "you have to be there" situations to fully grasp the process, but I hope this helps.