It questions the history created by those historicists including Marxist. For new historicists, old (traditional) history is linear or chronological. It says that the old history is exclusive, it does not include everyone, it rather includes only those who are already in power. It does not talk about those who are not in power.
New Historicism says that truth is constructed/fabricated/created. When power changes, the knowledge changes, and truth also changes. Therefore, the truth is changeable. For them, knowledge is independent of perspective. The western people produced a body of knowledge about non-westerners that is merely their perspective.
New historicism even questions materialism. Cultural materialism is a form of domination, but it does not use force. It is an indirect means of controlling the people. Cultural materialists distribute their culture, and make the people speechless.
For instance; American gives arm, medicine and economic support, and affects our internal behavior. Gramsci sees another effect of cultural materialism and says that it produces hegemony, the consent of the ruled to be ruled: ruled themselves agree to be ruled. People through hegemony internalize their inferiority.