I've seen them, they're pretty enough but they have a finite life span. They're also fairly picky about their environment in terms of light levels and temperature, but they work for a while.
The principle is simple, add enough controlled brine shrimp and algae along with the right bacteria to start a nutrient cycle and seal. The algae will eat the nutrients, the shrimp the algae and the bacteria the shrimp waste and the decomposing shrimp bodies, resulting in nutrients.
Sadly, like all of these simple ideas, they're fine while they remain in careful balance, but any changes outside tolerance in the environment they're in and you have a dead ball.