Partial hand amputation is by far the most common type of amputation worldwide. Nevertheless, regardless of their potential clinical and socioeconomic impact, battery-powered partial hand prostheses, namely, powered digits, have modestly progressed so far, and very few clinical solutions are available today. Here, we present a mechanical architecture, an alternative to state-of-the-art solutions, which exploits a high efficiency, non-back drivable mechanical transmission based on a face-gear pair and a miniaturized clutch. We took inspiration from the synergetic prehension approach proposed by Childress for whole hand amputation. The finger was equipped with a myoelectric controller and a tactile sensor able to provide users with discrete event sensory feedback. Measured speed (90°/s) and force (6.5 N) of the newly dubbed S-Finger proved comparable with those of clinically available prostheses. The design demonstrated to be compact and rugged enough to undergo a clinical viability test with two partial hand amputees, fitted with custom three-fingered research prostheses using the S-Finger. The subjects successfully completed several dexterity tests and gave relevant feedback for the development of a second-generation device. These results contribute to the increasing research endeavors in the field of partial hand amputation.