[JRTJH]

The tank sensors (actually probes similar to nails) send a signal to the "illuminated display" when something wet makes contact with the probes. If you can imagine a piece of wet toilet paper or a "chunk of solid human waste" laying across the horizontal probe inside the tank.........

The "illuminated display" will indicate where the highest "wet spot on a probe" condition exists...

Here's what the "nail in the tank" looks like. Keeping it clean so it won't sense "being wet" is daunting. If you're bored and have lots of time to spare, I suppose the effort (and frustration) to keep "nails clean inside a poo tank" is a challenge. Most just ignore the sensors and the "illuminated display" and just rely on how the toilet sounds when you flush it. When it "gurgles" you'll know it's time to dump.