The rule titled 'Unfamiliar Sign-In Properties' is designed to detect sign-ins that exhibit properties unfamiliar to the user, leveraging historical sign-in data to identify potential anomalous or suspicious access attempts. It monitors various factors associated with user sign-ins, such as location, device, or browser changes, that deviate from the norm established by previous behavior. This detection mechanism specifically tracks 'unfamiliarFeatures' under the risk event type, indicating that the user's sign-in patterns are being challenged by atypical properties. By identifying these unusual sign-ins, the rule aims to prevent unauthorized access, enhancing the security of user accounts against risks associated with identity attacks or credential theft.