The Omnium Portable Bike Trainer comes in two versions, the Over-Drive,  resistance model, and the Zero-Drive, non resistance version. 

The Zero- Drive has grey plastic end caps and the Over-Drive has black plastic end caps.

The Over-Drive version features Internal Progressive Resistance that utilizes the power of an eddy current. In short, the faster you ride the more resistance is applied, there is no adjustment to add or reduce the resistance, you simply use the gears on your bicycle just like outside. 

The Zero-Drive trainer offers no resistance. This trainer is great in cases of single speed bikes (such as track bikes) or for a very basic low exertion warm up or cool down trainer.