My understanding, and someone please correct me if I'm wrong, is that all the US index futures and exchanges have in essence the same circuit breaker policy - a one hour limit down after -10% a 2 hour after -20%, and a forced close after 30% with a 5% overnight limit for the futures. The thing is, none of that allows you to in any way cap your losses. There's no guarantee (as any commodity trader will tell you) that you won't have multiple limit down sessions in a row where the market simply open ask-limit down and sits there with zero volume or closes up shop for the session (depending on exchange policy). So in effect, your real risk is -100%. A fact known to any decent commodities futures trader is that limits do NOT decrease risk, they increase it by preventing the execution of stops. If you think slippage in a crashing market is bad, wait until you see slippage in a closed market. Limits favor institutional investors with the bankroll to fade the risk. The tighter the limits, the more so this is true.
1) That's better than a single, fixed, daily price limit. 2) If you can keep your wits and feel comfortable, you can trade options that are far enough out-of-the-money where those premiums themselves haven't locked at their limit and attempt to trade your way into or out of a market.
But what was the SPX at open? With ES closed for trading and SPY still trading, there is no arbitrage going on. So it's not a true market scenario with normal price discovery mechanisms in place
But the index was not the market price (due to trading and reporting delays, many stocks not even opening for trading etc). The market price (i.e. the price you could actually buy or sell at) was the index futures, which gapped down a lot more.