The Duke brothers tried to steal the crop report. But Billy-Ray and Lewis, after having been quite ill used by the Dukes, get the real report and give the Dukes a fake.
The Dukes send their trader to the commodities floor with instructions to buy like crazy. Their fake report shows a tough winter, and so they're betting the price will go way up after the "official" release of the report. The trader starts buying and others see the Dukes trying to corner the market, so they start buying. This drives the price up.
At just the right time, Billy-Ray and Lewis "sell" futures contracts for orange juice. They sell like crazy. They're selling contracts for orange juice they don't even own and can't deliver. (Basically short selling - in commodities all you have to do is be able to deliver at the date in the contract). Their selling drives the price down a bit. It also pads their trading account with a ton of cash.
The secretary of agriculture announces that the winter did not affect the crop, so the pit full of traders freaks out! There's not going to be price pressure, so they're all holding OJ contracts that are seriously overpriced. A mad selling frenzy ensues, driving the price way down.
Again at a point timed for maximum effect, Lewis and Billy-Ray Valentine announce that they'll "buy em" -- they buy back the OJ contracts at a much lower price than they sold them for earlier, netting them a huge profit. They also refuse to sell to the Duke's trader, freezing him out.
The Duke's get a margin call and go bust and our heroes make a lot of scratch. Of course, in real life, trading curbs and circuit breakers would prevent either side of this play, but it is educational and funny to watch.