Auto-Transformer: how to understand the following diagram?
If you applied 277 volts to the top-tap of the primary or instead applied 120V to the bottom tap, the voltage seen at the secondary tap point should be the same. In fact the secondary wouldn't know whether it was supplied from any of the primary taps because they should all be designed to produce the same voltage at the secondary tap point.

If, on the other hand you decided to put 100 volts on the top-tap you couldn't expect the correct secondary voltage to be generated.