Actually, Dai uses margin trading to sustain its value of $1. You can find the details at the link I posted. In general, you use your ETH as collateral to draw out Dai and you can use that amount of Dai to invest in risky ICOs or simply trade with other cryptocurrencies. Meanwhile, your ETH is still kept securely in a CDP smart contract. If ETH goes down in price, you can use Dai to pay back your loan and withdraw ETH at the equivalent value which is lower than your initial investment. For example, imagine at first it costed you 1 ETH to withdraw 194 Dai, but then ETH drops in price when Dai is still $1, you will only have to pay 180 Dai to get your 1 ETH back. ETH is not the key factor that maintains the value of Dai, but the economic incentives are.