When the laptop's running on battery power, a small icon appears near the address bar, allowing you to flick a toggle to turn the feature on.
The company has tested the feature on a Lenovo X250 laptop running Windows 10. In the experiments, it opened 11 different websites, scrolled through them a little, left the tabs open, did nothing for a minute, then repeated the process again. And again. Until the battery died. Opera claims that the battery lasted on average 1 hour 54 minutes being used in this way with Chrome, or 1 hour 58 minutes with Opera. With Opera's power-saving mode, though, it apparently ran for 2 hours 56 minutes.
Obviously, a test performed by the company itself is bound to provide a favourable result. And it's hardly the most rigorous of trials, being performed on a single OS and laptop. But it will be interesting to see how it performs in the wild. You can download the developer release to take it for a spin.