For the average user, there is no direct way to cash out your Tether for 1:1 USD. I imagine that big clients do have the ability to trade Tether for USD. Because of that, they will buy up discounted Tethers on exchanges and redeem them to Tether Company for 1 USD each.
HOWEVER, this clearly relies on Tether Company providing that exchange. If Tether Company STOPS allowing the big guys to do this arbitrage, then suddenly the house of cards would collapse very quickly. The longer Tether stays below 1 USD, the more pressure is being put on their company by arbitrageurs.
Furthermore, we know that Tether has made a loan to Bitfinex for $900M. The Tether Company therefore only has cash on hand for about 2/3 of the outstanding Tether, assuming they had 1:1 reserves prior to lending cash to Bitfinex.