This particular section of the text can be found in Chapter Nine, where Paul is in No Man's Land and in the middle of a bombardment. Suddenly, an enemy soldier jumps into the same shell hole he is hiding in, and Paul quickly stabs him. However, he is agonised to discover that the man is not dead but is still barely alive. This is the first time Paul has killed a man in hand-to-hand combat, and it is clear that Paul is shocked by the responsibility he feels and the guilt he is burdened by. Note how he describes his emotions as he watches the French soldier gasping out each breath:
But every gasp lays my heart bare. This dying man has time with him, he has an invisible dagger with which he stabs me: Time and my thoughts.
The quote is important because it describes the impact of the French soldier and his suffering on Paul, and each breath is described as exposing Paul's emotions and filling him with horror. Paul is having to face the consequences of his actions, and because this is the first time he has killed anybody like this, it is much more difficult. In addition, note the use of the metaphor in the "invisible dagger." Even though it is Paul who has just stabbed the soldier, it is the soldier who is described as stabbing Paul as he is filled with guilt. This is why Paul does what he can to save the soldier, even though he knows he is fighting a losing battle.