The command card "Brace for Impact" reads " Use while defending to add 1 black die to your defense pool."

Here is my assumption: You can play it any time up until "add defensive modifiers" step, but if you play it after dice are rolled, you can't roll the extra die you just added to your attack pool and so it does nothing. Therefore to use it you have to use it before dice are rolled.

Is this correct? Most of the other "add a die" command cards are "when an attack is declared" not "while attacking" or "while defending".

Don't assume, read the rules.

· Any ability that is resolved "while attacking" or "while
defending" can be used at any point during the attack with
the following exceptions:
-- An ability that adds dice to a dice pool can only be used
immediately before step 2 of the attack.

I guess the difference from 'use when attack is declared' cards is that the defender can use this after the attacker creates their dice pool and passes up the opportunity for power tokens.

The defender can also use Brace for Impact if the attacker adds an attack die through some ability.

Also, do figures that doesn't have a set set of dice for their attacks (such as IG-88 and General Weiss) have to settle on their choice of dice immediately before step 2 (as Brace for Impact) or only once you are in step 2. Because if they have the same timing as these cards, then conflict resolution becomes important and can lead to intersting twists).

Choosing your attack pool is an attacker ability, and it affects the dice pool, so (unless there is an explicit timing that activates it even earlier) it must be done just before rolling the dice, before defender abilities that affect the dice pool. The defender would thus have knowledge of the attack pool when deciding whether to use Brace for Impact.