Rule 21 compliance affects grid tie inverters connecting to the utility grid. The Rule been adopted in California but it appears that other states will do the same. Rule 21 requires an inverter has to respond to frequency shift. As an example, if the frequency of the grid is changed to 60.5 cycles per second, instead of 60, the inverter would put out no more than 50% of its rated power.
This turns out to be an elegant way for an electric company to control how much power your inverter is putting into the grid. One way to overcome this is to use a battery. Instead of putting out 50% less power, the power can be put into a battery.
Inverters use a phase lock loop to run in parallel with the grid. Voltage is slightly higher from an inverter so that electricity can be transferred from the inverter to the grid. Wiring and circuit breaker resistance means a very small higher voltage would be necessary for electricity to flow from the inverter to the grid. Parallel circuits exist between grid loads and loads in your house. Grid tied inverters make current flow because the grid seems like a load to the inverter