Question

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

Answer 1

The answer would be the sandbag wall.

Stacks of sandbag will be put in front of the trench to protect the soldier. When the soldier stands inside trench all of his body is covered. But when he stands on trench board, the height increased so that the chest might be exposed. Sandbad will be helpful to stop the bullet that aimed at the chest region.