Hyips in low-risk category claims that they generate revenue from something such as advertisement sales, banner sales, forex, poker bots etc.
That is why we call them low-risk hyips as they are safer then pure hyip/ponzi sites which are in high-risk category. But still, those revenues may be lies of the sites to attract more investors. You need to be careful while trying to make money online is these sites.
Still, you can give these sites a try, they are better than high-risk hyips.