Rivest, Weitzner, and colleagues developed the design for the smartphone-based system. The approach uses inter-phone Bluetooth communications (including energy measurements) as a proxy for inter-person distance measurement. Through some clever applied cryptography and architecture engineering, this system can be used to collect and maintain weeks of contact events which can later be enriched by infection notifications (as specific individuals test positive) leading to exposure notifications to all cell phone owners who have had medically-significant contact (in terms of distance and time) with infected people in the past medically-significant time period (e.g. two weeks).
All of this can be done without revealing any private information to anyone, except when an infected person chooses to reveal their own identity and personal location trace to public health authorities.