So in a power grid, producers (powerplants, windfarm...) offer how much energy they can put into the grid for a certain time of day, whereas consumers, e.g. your energy provider, which manages many households, or the industry go to the energy market and say: "I need this much energy at this time of day".

So now you have to make sure this supply demand evens out.

Say for example there is more demand than actual supply, then positive operating reserve has to be put into the power grid (e.g. turn up your gas power plant). For these smaller fluctuations of supply demand you have to have something which scales quickly. A nuclear power plant is probably not so easy to scale and the output is rather constant.

Now the other case: There is not enough demand and too much energy is produced in the power grid. The energy needs to go somewhere. So now you have to find a consumer, who can quickly scale up it's demand. This might be a flour mill or here it would be AWS. And for offering that you can consume an energy X in the next hour, you get paid by the Department of Energy, whose incentive is to keep the grid stable.

So AWS would monitor the energy market and would have to know, how fast their customers would turn up more Spot instances, once AWS offers them a lower price. Then they would know how fast they can scale their energy demand in the data center and make an offer on the energy market that they can consume an extra X MWh in the next 30 minutes.

So fast scaling up demand for consuming energy from the power grid would be "negative operating reserve".

If you are just a consumer and have a consistent demand, this would not be "negative operating reserve"