Every time a trade happens on ShapeShift, you have a chance of winning a reward! The more FOX you have in your wallet, the higher your chances are of being selected and winning USDC!
Rainfall is completely autonomous and while the winner is selected at random, you can boost your odds by holding (HODLing) FOX Token. You will need to accumulate at least 10 USDC in winnings before you can claim your reward. Steps on how to claim your reward can be found here.
The official rules can be found here.