It involves a tether providing precisely-timed tugs:

"“When we walk, there is a short period between steps where one foot is stopping its forward motion while the other is preparing to accelerate to take the next step forward. Our research shows that this brief window where both feet are on the ground is the best time to apply force to assist walking most efficiently,” said Philippe Malcolm, Ph.D., assistant professor in biomechanics at UNO and lead investigator of the study.

The device works by providing timed pulls from a motorized pulley while an individual walks on a treadmill. Since it only requires wearing a waist belt, it is relatively easy to make individualized adjustments compared to more complicated devices."