When you get through the main part you will see a plane wing that looks like the Nacht der Untoten sub-titles.

When you have to get the bazooka to destroy all of the tanks, there is a man just in the bunker to the left side of you.

Then get back and as far as you can (once you have the bazooka) and aim for the man's waist. Then press RT to fire at him.

Then what should happen is his leg will shake in mid-air which is given the name 'phantom foot' (note this doesn't happen every time).