The power plant on the left shows two generators tied in synchronization.
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The power plant on the right shows two generators not synchronized.
At the instant when the lamp in the center goes dark, it's the time when synchronization is achieved and the generator can be tied or linked electrically together to carry a bigger load than a single generator can handle.
The reason for the center light to go dark when in synchrony is because there is no difference of potential between phases and this can be seen on the scope with the sine waves perfectly overlapping each other.
