Stages Indoor Cycling bikes include an easy-to-use resistance knob that adds resistance by rotating the knob clockwise, which adjusts an "eddy current brake", which is two large magnets positioned near the flywheel that add and remove resistance based on distance from the flywheel. This allows for a smooth, silent, frictionless resistance system.
In addition to the resistance knob, the SC2, SC3, and Solo models also include the proprietary SprintShift lever, which allows for rapid adjustment of the resistance to consistent points. The SprintShift levels will vary based on your resistance starting point - so once you have set the resistance with the lever to the left, you can turn it to the middle and far-right positions to add more and then even more resistance for your efforts, while returning to the far left position for recovery. This ensures you can always return to the same levels of resistance for your workout easily, while still allowing for use of the infinite micro-adjustments of the resistance knob.