The phrase "knocks props out" refers to something that undermines or weakens a person's argument or position. Antonyms for this phrase might include "bolsters," "strengthens," "supports," or "upholds." These terms refer to actions that reinforce or validate someone's point of view rather than undermining it. If you want to express the opposite of "knocks props out," you might also use phrases like "affirms," "confirms," or "validates." Essentially, any term or phrase that indicates that something is being upheld or confirmed could be considered an antonym for "knocks props out." These words and phrases can be useful when you want to express support for someone's argument or position.