We have some unexpected things that happen now and then in Chalo Chalo, we call them rarities. They're designed to foster a sense that the game world has hidden depth and mystery, and to keep players on their toes.
Chalo Chalo is built to include a concept of 'rarity pressure'. You can think of it as 'something in the air' that influences how likely strange things are to happen. The higher the pressure, the more likely that a rarity will manifest itself. When nothing out of the ordinary is happening, the rarity pressure is incremented after each race. We want rare things to be more likely if you didn't see any for a while, and less likely if you did. so to complete the scheme, each rarity has a 'rarity_pressure_release' value. The rarity pressure drops by this amount after the rarity manifests itself.