The simplest answer I can give is that assets place bids and a volume of energy that they are willing to turn down if the system operator needs to. Those bids are either positive or negative in value and this depends a lot on the type of asset, for example wind assets usually bid negative (ie. we pay them to turn down) while gas assets usually bid positive (ie. they pay us to turn down). The reason for that is a lot to do with complexities of the market and also the cost of running that assets, the cost of fuels, etc.
