The power company themselves sends the signals.
This a a "baby step" V2G implementation that we will see increase over time. California used to have brown outs during hot summer days, but with the high rate of solar adoption, they now have over capacity during daylight hours (and they would actually like you to charge your car then). The peak time comes after work when people get home, turn on the lights, fire up the oven or stove, crank the A/C, etc. Power grids have to be designed to meet this peak load, so that is when the natural gas, coal plants, etc really need to be cranking. The problem is that these are not easy to fire-up "on demand". Using batteries to store the over capacity produced by solar (and to some extent by wind) during the day for later use allows a grid with lower overall power generating capabilities.
This buffering by batteries will be the primary use for "worn out" EV batteries that still have 70% of their original capacity. Not good for driving your EV, but perfectly adequate for grid buffering.
V2G (Vehicle to Grid) technology allows EV's to be used to buffer the system and be part of a bigger battery storage system on the grid itself. It allows the utility to "borrow" electricity from an EV that is plugged in and connected. This is done at a low rate, so doesn't stress the battery and reduce battery life. Heat is the primary cause of battery degradation, and slow charge/discharge produces little to no heat. The EV is then "paid back" the electricity when grid demand is reduced. You will likely get "paid" a small amount to allow the utility to do this.
For more info and graphics, google "Duck Curve".
