Ask HN: How will all wind and solar generation maintain grid frequency?

Then it free-runs at about 60Hz (50Hz). It only has to synchronize with the grid if it is exchanging power with the grid.

The Old School way of synchronizing a generator with the grid is to connect a light bulb between each leg of the two disconnected grids and then adjust the generator speed until the light bulbs extinguish. As the two get closer to synchronization, the light bulbs will go bright and dim slower and slower. When they go out, you close the grid connection switch.

When the switch is closed, adding power to the generator (e.g. gas to the engine) doesn't significantly affect the speed of the generator because the grid absorbs the power (conceptually it is "pushing" energy into the grid). If you reduce the motive power (e.g. throttle down the engine), the "generator" will effectively become a motor and will keep the engine spinning at the same speed.