It actually works the other way around. What is displayed in the discarded cards is derived from hand before and after played card and newcard flags. If you look at
Whirlwind code, you'll see this:
$newcards = array (1=> 1, 1, 1, 1, 1, 1, 1, 1);
$mydata->NewCards = $newcards;
This explicitly sets newcard flags to all cards in your hand.