The machine is, essentially, a high-intensity stair stepper and rowing machine fused together.
From the dailynews.com
This turns the stepper motor into a high pole count brushless servo motor, with exceptional low speed torque and position resolution.
From the en.wikipedia.org
The stepper motor shaft is also connected to a high resolution optical encoder for position feedback.
From the techcrunch.com
Stepper motor systems must thus generally be over engineered, especially if the load inertia is high, or there is widely varying load, so that there is no possibility that the motor will lose steps.