On the D-Day level, after you get off the boat and climb up the cliffs, go to the bunker immediatly to your right (the round one), then go to the bottom step in the bunker.
Go into the prone position (lay down) and look in the same direction as you did when going toward the bunker
Crawl straight into the bottom step and you should go straight through the step. Keep going untill you fall down the ledge.
You then appear under the whole battle field and can take out enemies from underneath while they can't shoot you.