Flash Sales are promotions that offer a special rate for a very limited period of time. There are various channels that sponsor flash sales, ranging from the common "Limited Time Offer" on booking.com to companies specialized in this activity, such as Secret Escapes.
Flash sales have a unique appeal, driven not only by the highly advantageous rate but also by their "temporary" nature: if a customer wants to book the discounted rate, they have a short window of time to make the reservation, and availability is limited. In fact, it's possible to dedicate only one category of rooms to the flash sale, and some sales channels even allow the promotion to be offered for just a few units. Essentially, this "neutralizes" the sense of ease customers experience when booking during the low season, using the principle of scarcity combined with a highly competitive rate.