You have to break a peice of their equipment before you even start worrying about it. Then you have to go on a severe offensive, hopefully they block a ton. That drains their soul gauge, the sphere next to their life bar. You want it red. You want it blinking red. You want yours blue. You want it blinking blue/white :]

After both of you are blinking and their armor is broke. You have to hit them with a power attack that breaks their block (soul crush). THEN as SOON as you break their block you hit LB or L2 (or all four face buttons). If you've done it right the critical finish cut scene happens