Transformer synchronization
Imagine what would happen if the transformer was reconnected while the loads on the secondary were still spinning down and no longer in phase with the grid supply.
Reconnection out of phase would be a Bad Thing : I hope you already understand (or the book explains) why.
So it would make sense for there to be a secondary means of disconnection to prevent that.
Also remember that 40% of rated voltage on one winding necessarily implies 40% (to a good approximation) on the other winding, so the answer to the last question is yes.