Once per round, choose any number of attackers dice and make him reroll them.

Powerful really. But do you do this after their dice have been modified with focus or before?

I think yes. It must be write "immediately" if it can be use only BEFORE spending focus. There are upgrade with the word "immediately" like Heavy Laser Cannon so i think they work different.

The defender has an opportunity to modify the attackers dice and that occurs right after he has rolled them, before he modifies them.

Yea spot on. Just read it. Clear as day in the rulebook. I think its just a case of it so rarely could come up before but these options are now very viable. Cheers

Sorry didn't know it, my answer was wrong.

Worth nothing though that the rule for "one reroll per die" still counts even if the defender is forcing the reroll. Let's say your opponent rolls 3 hits, and you force him to reroll all 3, and gets 2 blanks and a hit, your opponent may not use a TL or other ability to reroll any of those dice (but they may still use focus or other non-reroll modifiers)