You put pressure on Favre with the front 4 and drop back people into zone and Favre will throw it up for grabs.
I'll never understand why more people don't exploit this. Favre hates to take sacks (and I would argue hates to be hit too) and will chuck it if people get in his face. He's also not good at reading zones, never has been.
It's not just the Bears. Favre had trouble with the Bucs back in the day and the Rams when Lovie was coach. That's the main reason we hired the guy.