You can't power down the whale's SP, of course. Neither can you delegate it forward. It is indefinite, up to the whale. But they can cancel it any time like I mentioned above. The system only considers the delegated SP as yours when you vote.
The delegated SP is also shown separately in your Steemit.com wallet under brackets. For example, you can see here - https://steemit.com/@curie/transfers . The account's owned SP is 38k SP, while the delegated SP is 333k.
thank you so much for the explanation! It makes things very clear now :-) It's like a 'boost'!
Boost, I like it! Sounds better than delegation :)
It could be called a PB...Power Boost!