What I'm assuming they mean is that if you have 100 BitUSD that is equal to $100 USD. And if BitUSD dropped in value to say $90, your bitUSD will fluctuate to 110 or something to compensate the change. So the coins/asset are the #'s that are actually moving not the value.
Now the question I have is say if you have 1000 BitUSD and you want to cash out to USD. Who is going to be the one giving you actual dollars? Sure if you put in $1000 USD and purchased 1000 BitUSD. Say if you wanted to sell that 1000BitUSD now, who is going to buy that back from u at $1000?
I don't think so. If I understand it correctly, 1 BitUSD is backed by enough Bitshares to equal $1. Someone has locked up 3 times as many bitshares as would be needed to collateralize the BitUSD, so assuming there isn't a quick and devastating crash in bitshares, there should be enough collateral to make you whole. So 100 BitUSD would always be worth $100 and would not fluctuate.
At least until there are gateways available, no one will give you USD for it. If you decide to cash in your BitUSD what you will actually get is enough bitshares to equal the value of USD at the time you decide to cash in. You would then need to sell your bitshares on an exchange to get actual USD.