A Carol Bike is a smart stationary bike designed to deliver quick, high-intensity workouts using a method called Reduced Exertion HIIT (REHIT), which involves short bursts of intense pedaling with minimal recovery time, claiming to provide significant fitness benefits in significantly less time than traditional cardio workouts; it features AI-powered resistance adjustments to personalize workouts based on your fitness level and is marketed as a time-efficient way to exercise, with sessions often lasting only a few minutes per day.