Although this technique has been around a while, I had not heard the term until recently. Flash Détente. Sounds more like a French superhero or some negotiation where one party simply walked away from the bargaining table.
As I understand it, grapes are added to a very warm silo-like device and the heat evaporates some water from the grapes. They become more concentrated, the must is a deeper color and it can even remove some of the green, bell pepper notes from under-ripe grapes. No one does this with healthy, ripe grapes but it apparently corrects in cool/wet vintages.
One can not help but wonder what the exposure to 180+ degree heat does to the grapes in the longer term but since most people age their wine less than a day after purchase, I guess we may never know.
The process differs from the vacuum concentrator which allows a similar result to be achieved at a lower temperature. I have tasted wines made using this technology and it seems to be a gentle enough method. I have to assume it is more expensive than Flash Détente or else why would you choose heat?
I wish there was some labeling required for wines handled with this sort of manipulation. Not so much so people could avoid them entirely but so they would be aware and could make an informed choice. One might wish to know the added expense involved or simply to know that the grapes had been manipulated more than other wines.