I would gladly attempt to explain it again, but I will do so by way of an analogy that takes the blockchain out of the picture and replaces it with trusted 3rd party.
Assume 1 BTS == 1 USD on the market as the initial condition.
Sam wants to buy 1 BitUSD
Pam wants to short 1 BitUSD
Neither Sam nor Pam currently own any BitUSD because BitUSD is issued by Bank of Dan.
Before Pam can sell BitUSD she must borrow it from Dan. Dan doesn't trust Pam one bit so requires 2x the collateral to be held in escrow. Pam agrees and gives Dan 2 BTS and Dan lends Pam 1 BitUSD.
Pam then Sells the 1 BitUSD to Sam for 1 BTS.
At some point in the future Pam has to pay off her loan from Dan and to do so she must buy 1 BitUSD on the market.
There are two outcomes:
1) 1 BTS == 2 USD, Pam is able to buy 1 BitUSD for 0.5 BTS on the open market, then go to the bank pay off her loan and free her 2 BTS collateral. In this case Pam ends up with a profit of .5 BTS for a 50% gain.
2) 1 BTS == 0.67 USD, in this event Dan starts to get concerned that Pam will run out of margin so exercises his right to cover the loan with the collateral. Dan enters the market and buys 1 BitUSD for 1.5 BTS and then gives Pam the change of 0.5 BTS minus a margin call fee.
At the start and end of the day all BitUSD is owned by the Bank.
BTS can be thought of as shares in the bank. Dividends as profits from operating the bank.
The only thing required for this to function is a way to enable the block chain to enforce margin calls that cannot be manipulated.