I think at this scale that it's cleaner to think of the grid as a flywheel system of enormous (~infinite) mass. Every consumer (home, factory, business, whatever) and every producer (home solar, nuke plant, wind machine, whatever) is connected to this flywheel system, and they're all in parallel (just like circuits in a home are).
Take power out of the flywheel? Sure, why not -- that's exactly what we've done with it at home since we had an AC grid to speak of.
Try to impart higher voltage to put power back into the system? Fine, no worries. Flywheel just accepts and absorbs whatever you put into it as long as it has the right shape.
Try to change the flywheel's AC frequency or waveform, though? Hah, fat chance: Flywheel will trip your breakers or reduce your gear to ash -- whichever comes first. We play by the flywheel's rules, and those rules aren't optional at all. It may be just a big dumb flywheel, but the flywheel's enforcement is absolute.
Anyway, it's a big mythical flywheel. We get charged money to take power out of it, and (with the right arrangements) we get paid money to put power into it.
And by design, grid-tied inverters are quite good at following the flywheel's rules. They match the flywheel's waveform very well [lest they go poof]. They can only put power into that flywheel, and they reject power from that flywheel. The inverter is a one-way thing that, from a high-enough viewpoint, looks kind of like a check valve or a diode does.
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But it's not an ideal, perfect system. It does not exist in a world of frictionless pulleys and ropes that don't stretch.
That imperfection shows up in the wires and transformers connecting the house to that flywheel system, in that these parts have a non-zero impedance. (For the sake of this discussion, impedance can mean about the same thing as resistance)
And that impedance is useful. It means that we can increase voltage a very tiny bit on our local leg, and thereby succeed at using our puny little solar power system to help cover our own local base load.
Suppose our plug-in solar widget can produce 800 Watts today. The plug-in kit increases the voltage that it drives that plug with until 800 Watts of power output is achieved (which is only a very tiny amount of extra voltage, but it's real anyway). This is its purpose: Where some robots pass butter, this robot puts 800 Watts into a power socket.
Since the wiring between the plug-in solar and the local household baseload stuff has a lower impedance than the path to the mythical flywheel does, our baseload stuff (the fridge, the router, the fan by the window) gets first dibs at that 800 Watts of minutely-enhanced voltage.
If we produce more than we use, the excess local power production gets provided to and soaked up by the flywheel (maybe we get paid for that).
If we use more than we produce, then the deficit is made up for by the flywheel (and we definitely pay for it).
If our generation and consumption is perfectly balanced (which never really happens), then the flywheel plays no real part in the program except to provide a waveform for our solar inverter to synchronize with.
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Anyway, it's not new tech at all. It's safe in that it won't work at all without a connection to the big flywheel in the sky. And folks have been using grid-tied inverters for quite a long time, but that was done on a dedicated circuit (with a dedicated breaker or fuse or RCBO or whatever they're called wherever you are).
But there's still some new stuff going on: It can be a plug-in system that works without individual planning or permitting; in some places, a person can legally buy and use a kit without obtaining extra permission.
The safety of sharing a circuit is also important, since enhanced potential exists for overheating things. Germany limits their plug-in solar to 800 Watts, or about 3.5A @ 230V, while their circuits are usually 16A.
That means there could be as much as 19.5A of nominal current flow within a circuit before some combination of circuit breaker or inverter limit is reached.
I learned a long time ago to never question a German about their electrical standards, though, so I can only assume that very smart people have worked very long hours to see that this situation is safe-enough.