There are actually two points of contact when Nymi is in use. You push the top of the sensor (the side that's like the face of the watch) with a finger from the opposite hand. The underside of the sensor touches your wrist. (http://www.getnymi.com/)
It appears the technology behind this device is "HeartID". (http://www.bionym.com/news/ - "See HeartID in action, enabling finger-touch based user authentication.")
This technology is described in (Fatemian et al. 2010) and in (Agrafioti, 2011).
One patent describing this technology is (Agrafioti, 2012).
Uniqueness of heartbeats
Quoting from the patent application (Agrafioti, 2012):
Uniqueness - while different signals may appear to conform to the same patterns, there is large inter-individual variability, resulting from different physiological parameters controlling the waveforms, and physiological factors (e.g., heart mass orientation, conductivity of various cardiac muscles, and cardiac activation order) can introduce significant variability among subjects (in fact, significant medical research had long sought to reduce this variability for universal diagnostic standards)
From the dissertation (Agrafioti, 2011):
Uniqueness is guaranteed in the ECG signal because of its physiological origin.
While ECG signals of diﬀerent individuals conform to approximately the same pattern, there is large inter-individual variability due to the various electrophysiological parameters that control the generation of this waveform.
The patent describes collecting the ECG signal from both wrists, but using one wrist and the opposite finger should have similar effect.
(Agrafioti, 2010 at p. 91, Table 4.2), the error rate of their identification method at that time is shown.
Depending on the individual, equal error rates between 0 and 23% were achieved. The strictness of the classifier can be tuned depending on the target application to either be more lenient and allow more false authentications or be more strict and fail to authenticate the true individual more often. How they will choose this threshold for the production device, or if they will improve their classifier is not public knowledge.
Agrafioti, F. (2011). ECG in Biometric Recognition: Time Dependency and Application Challenges (Doctoral dissertation, University of Toronto).
AGRAFIOTI, F., BUI, F., & HATZINAKOS, D. (2012). WIPO Patent No. 2012151680. Geneva, Switzerland: World Intellectual Property Organization.
Fatemian, S. Z., Agrafioti, F., & Hatzinakos, D. (2010, September). Heartid: Cardiac biometric recognition. In Biometrics: Theory Applications and Systems (BTAS), 2010 Fourth IEEE International Conference on (pp. 1-5). IEEE.