Benefit: When you flank a foe with an ally who’s a different size category than you, you can reroll one damage die with a result of 1. To gain this benefit, the foe can’t be larger than the larger of you and your ally, or smaller than the smaller of you and your ally.
Teamwork Benefit: If your ally also has this feat, you’re both considered to be flanking an enemy as long as you both threaten that enemy, no matter your actual positions.