> Apparently to match the demand at given instance electricity plants rotate their turbines a bit faster or a bit slower instead of switching a complete plant on and off, which results in slight deviations from the 50Hz standard but it is fine as long as it stays between the limits.

This is slightly imprecise. It actually works the other way round: the demand is "communicated" through the network by slight changes in the frequency.

This can be described best for the example of steam-based generators, as they are typical for the majority of coal/gas/oil/nuclear power plants. When demand for electricity grows, for example because a new factory is powered up, the turbines in the power plants are experiencing larger forces attempting to slow them down, starting with the power plant closest to the new factory.

The speed (and thus the frequency) is constantly measured, and if it drops, the steam valve is opened just a tiny bit more to counteract the drop. If this is not possible because the power plant is already at maximum output, either other power plants have to take the load (and the electricity is then routed through the network to the factory) or additional power plants may have to be added to the network.

The important thing is: the frequency change is a means of communication, it originates from demand changes, and because the entire network is kept in sync, it works to communicate demand changes across the entire network. Power plants providing energy to a sub-part of the entire network always attempt to run their turbines with the exact same speed of 50 rotations per second, which should ideally always conform to the frequency in all other sub-networks that a specific sub-network is attached to.

If just one of these sub-networks does not perform its duty of re-establishing 50 Hertz by powering up their own plant output, the rest of the network must either channel enough power into this sub-network to allow its local generators to re-establish 50 rotations per second by taking some of the load off of them or - if that is not possible or decided against - must also deliberately drop their frequency (and thus the rotational speed of their generators) to match the sub-network that is deviating.

The only alternative to this would be to drop off the deviating sub-network entirely, but that is usually only done in extreme cases of deviation.