It's a bit more complicated than that, at least if I'm understanding the write-up correctly. The underlying trick is that they reduced the amount of yUSDVault s

It's a bit more complicated than that, at least if I'm understanding the write-up correctly. The underlying trick is that they reduced the amount of yUSDVault shares by redeeming most of them, then doubled the value of each by gifting more of the underlying yUSD to the contract. This in itself didn't conjure anything from nothing - each yUSDVault share was genuinely worth double the yUSD as before. The trouble is that due to some fundamental design flaws in C.R.E.A.M., this meant that a much larger Cream USD holding in one account and an offsetting debt in a second were now instantly worth double what they were before. The debt was then worth more than the collateral and could just be walked away from, whilst the other account had a profit which could be liquidated to wipe out C.R.E.A.M's reserves. Actually exploiting this in practice seems to have involved a whole bunch of additional fiddly details and transactions.