hmm, there maybe others that can explain it better because I only really got this myself recently.
Its based on "Derivatives" (a hedge fund staple) which is where one person buys a futures contract at a (hopefully) low but guaranteed price. This lower's the profit for the seller but also removes any risk. The buyer of the contract then hopes to make money when the future price rises (taking 100% of the risk of the price falling).
Sounds simple enough, but nobody makes money off of something simple
So to make things complicated hedge funds offer "Index" derivatives. This is were they hold derivatives contracts in exactly the same proportions of an index. This is so that whenever all the derivatives are lumped together, the summed value of the derivatives moves exactly like the index (because the index is based on the sum of the value changes of all its components)
Basically all this does is allow traders to work with one number (index) when determine the value of their index derivative.
So here you can see what a big deal it was that Goldman and Sach's did. By changing the Unleaded Gas portion of the index from 7.98% to 2.31% they caused everyone offering a "Goldman and Sachs Commodity Index" derivative fund to
Sell 70% of their unleaded gas contracts so their Index derivative now matches the Commodity Index.
Obviously this large of a sell off will have a big impact on unleaded gasoline futures prices.
As for Amaranth's disaster, its was the 20% change (and corresponding sell off) in Natural gas that caught Brian Hunter off gaurd. He's a 32 year old Canadian from Calgary that probably had inside info on Nat Gas supplies and why they let him get so heavly invested ($10bn us) in a single commodity.
anyways, I'm sure the shake out from all of this will be over sometime after the elections and Gas prices will go back up again