Penetrating Strike Clarification

[Angry Bob]

A seventh level rogue has Penetrating Strike. He flanks and shanks a mummy. Does he do +2d6 or +2 damage to it?

I told a friend it was the latter before realizing I didn't quite understand the wording.

[Apophis]

I believe it means that you roll half the dice you normally would, so 2d6.