Look at it in simplistic terms. If you own all the cotton in the market, you decide the price where you sell.

Lets say you corner 90% of the market by buying from cotton producers. They'll deliver to you the cotton and locked in their sales. Textile manufacturers need to buy cotton, so they go to the futures market for physical delivery futures and start buying. But if you're not selling the 90% of the market you've already bought, then all the textile manufacturers who need cotton for their production have to buy it from the 10% remaining suppliers who're selling futures. This creates a massive amount of demand with minimal amount of sellers that can deliver physical delivery cotton. This drives up price enormously since textile manufacturers can't afford to stop producing their product and cease business. Once the price gets driven up high enough, you now start selling to all the desperate manufacturers who're trying to keep business running and paying exorbitant amounts. You now can offload the 90% of the market slowly as demand stays pent up with no one but you to supply it (since the other 10% already sold while the price was coming back up)..