Challenging the credibility of the evidence or testimony provided by the accuser. California Code, Penal Code - PEN § 286 - Codes - FindLaw

Charges may escalate to "straight felonies" with longer prison terms (up to 13 years) if force, fear, or intimidation were used, or if the perpetrator acted in concert with others.

Specifically, this often refers to instances where a victim is deemed incapable of giving legal consent. This can be due to a mental disorder, developmental disability, or physical disability that is known (or should be known) to the perpetrator.