The side sensor is a patent-pending clinical-grade fingertip sensor, usually found in medical oximeter devices. This side sensor is at the core of the high precision in Watch 2. It delivers non-invasive and safe measurements of APG (Vascular Aging measurement), Blood Pressure and heightened accuracy to other measurements as well. 

Please note: 

• Your index finger must be clean and dry, otherwise it may affect the accuracy of the measurement.
• The APG service is only available if you have an active monthly subscription.