The way I understand it is that they have 2 means/modes of using their blood. Blood Arteries and Blood Veins. Blood Veins is the defensive one where their defense and reaction speed increases, while Blood Arteries is the one where they can perform techniques that can penetrate the reiatsu of a bankai.
When Opie got upset to the point of attacking Ichigo, he went into Blood Arteries, so Urahara was able to one-shot him.
Now, I hope Urahara really has figured it out. I guess the trick is forcing them to a state where they have to attack or you yourself go into a state where you can counter an attack without giving them a chance to go into Blood Veins.