Using restrictions (minimum length of stay, prepayment requirements, non-refundable payments, etc.) to filter out reservations on dates and/or room types which have excess demand. In general, no discount rate should ever be offered without fences, like advance purchase required, minimum stay length, non-refundable, etc.
An example of how to use them:
If all your business travellers are paying $100 per night, and book within five days of arrival and you need to encourage some extra, non-business revenue, you may want to offer a $79 rate for guests booking two weeks in advance. This advance purchase requirement is a “fence” that keeps the guests already willing to pay $100 from buying your rooms for $79.