How does the Monk’s Crushing Blow feat work?

The Stunning Fist feat says:

You must declare that you are using this feat before you make your attack roll (thus, a failed attack roll ruins the attempt). Stunning Fist forces a foe damaged by your unarmed attack to make a Fortitude saving throw (DC 10 + 1/2 your character level + your Wis modifier), in addition to dealing damage normally. A defender who fails this saving throw is stunned for 1 round (until just before your next turn). A stunned character drops everything held, can’t take actions, loses any Dexterity bonus to AC, and takes a –2 penalty to AC. You may attempt a stunning attack once per day for every four levels you have attained (but see Special), and no more than once per round. Constructs, oozes, plants, undead, incorporeal creatures, and creatures immune to critical hits cannot be stunned.

The Crushing Blow feat states:

You can make a Stunning Fist attempt as a full-round action. If successful, instead of stunning your target, you reduce the target’s AC by an amount equal to your Wisdom modifier for 1 minute. This penalty does not stack with other penalties applied due to Crushing Blow.

I’m not sure how this all works. Crushing Blow specifies Full-round action, so I thought that it was just the Fort Save for the enemy, and then you’d get the effect, or you wouldn’t. Since there is no normal attack, that can’t cause failure either, but you also can’t deal any damage.

During play, I found out others didn’t agree. Their interpretation was that I still had to make a normal attack, and then if hitting attempt the Crushing Blow – sort of like a different effect option for Stunning Fist that prevents Move Actions?

So how do I use this feat? By itself or as part of a Standard Attack hit?