Simulated market, simulated book. No wallet, no chain, no funds. Ratios keep stepping through the same mean-reverting process the rest of the site draws, so positions move and reversion actually pays out.
Nothing open. Sell the rich side of a pair that is two sigma out, or supply a range around one that is sitting on its mean.
Selling the rich side is short the ratio. Nothing is borrowed, so the position cannot be closed for you.