The reluctor ring is wrapped around your rear axles near the wheels. A proximity sensor reads the teeth on the ring and compares it to the signals coming from the other tires. If there is a discrepancy, the system determines that one of the tires is slipping and activates the ABS.
There are no reluctor rings on the front tires. Instead, a sensor in the knuckle actually senses the ball bearings in the front wheel hubs.