1) Someone who wants to sell $1M USD of BTC may be happy to take $1M USDT instead for legal/tax purposes. This transaction would have zero impact on the BTC/USD price.
2) Tether receives $1M worth of BTC they can slowly disperse back into the market via their own exchange (since Bitfinex owns Tether) in a controlled manner that doesn't push the price down.
3) With USD in hand they can either put it into their reserves (assuming we trust them) or they could also use the cash to further eliminate any selling pressure on the exchange (with a large buy order "wall") or even generate upward pressure.
4) BTC price becomes stable or only moves upward and attracts more outside money and speculation. Repeat the cycle.