Flash venting (aka crash cooling) is opening the lid of the kiln on cooldown to allow the heat to escape, thus, cooling the kiln more quickly. In most instances, it is not necessary and some people claim it is hard in the kiln.
You usually flash vent to stop a heat process, such as when doing a drop vase or a drape. When the drop or drape gets to exactly where you want it, you cool the kiln to stop the glass from moving any further.
Jester’s Baubles Fused Glass Designs