Perhaps, the blogpost is more accessible than the original paper:
In the beginning the reserve is empty. You cannot buy Djed, then, because there would not be enough ADA in the reserve to be in the ×4 – ×8 range.
So, first, someone has to buy Shen, first, to fill up the reserve. We don’t have a price for Shen now, so the protocol has a minimal price as parameter. Let’s say it’s 0.2 ADA/Shen.
User A buys 1500 Shen for 300 ADA. Now, we have 300 ADA in the reserve.
Now, if the current exchange rate is 1 ADA/USD, user B can buy 50 Djed for 50 ADA. Then, the reserve has 350 ADA, 50 ADA are liabilities for the 50 Djed of user B and 300 ADA are the equity for the 1500 Shen of user A.
If ADA falls and we now have 2 ADA/USD, liabilities would be 100 ADA for the 50 Djed of user B and equity would be 250 ADA for the 1500 Shen of user A. So, the Shen would have fallen to 0.16 ADA/Shen, but, since 350 ADA reserve divided by 100 ADA liabilities is only 3.5, user A could not sell their Shen, anyway.
If ADA rises and we have 0.5 ADA/USD, liabilities would only be 25 ADA for the 50 Djed of user B and equity would be 325 ADA for the 1500 Shen of user A. So, the Shen would have risen to 0.216 ADA/Shen and user A could sell a lot of it, since 350 ADA reserve divided by 25 ADA liabilities is 14, so there is more than enough reserve.