They're saying that Stillman Valley (out by Byron on 72) had a confirmed twister.
As to how they (sump pumps with backups), work, is usually the secondary pump is mounted a little higher than the main electric pump, so if water is coming in at a rate that the main pump is on all the time and the water level is still increasing, the back up pump (which is still under electric power as long as power is present) will engage. Mine has a separate float not directly connected to the pump like the main pump. As such, I have that mounted just above the main pump. So should my above scenario happen, the battery back-up pump will engage (on electric power at that time), and then the tender will let me know it engaged.
Also, many people (Myself included), if you have a finished basement with a bathroom down there, you will most likely have an ejector pump (a sump pump). I suppose if water can get in there, that would also engage.