What prevents bullets striking a soldier's chest when he is on the trench board (fire step)?

The sand bag wall prevented soldiers from being hit in the chest by rifle fire when stepping onto the fire step to shoot at the enemy.

The trenches of World War I were not so deep that the soldiers were entirely protected by the trench itself.  They would stack sand bags along the top of the trench to form a protective wall.  The sand bag walls were effective against rifle fire, but not terribly effective against artillery fire from the opposing side.

Trench warfare in World War I was miserable and gruesome.  Check out All Quiet on the Western Front by Erich Maria Remarque (1929) for first-hand descriptions of the misery of the trench warfare.