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Were the market shutdowns manual or automatic handbrakes? I know there are breaks when specific thresholds are hit that are pre-determined.

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As far as I know, they were automatic handbrakes based on excessive volatility. A feature designed to prevent chaos and people from over buying/selling due to panic.

Brokerages preventing users from buying certain stocks is clearly manual, and fairly unprecedented afaik

Yes. Both things happened. I was specifically talking about halting trade multiple times during the day.

There are different layers here.

Baked into the exchanges (NYSE, Nasdaq) are circuit breakers which halt all trading on certain volatility conditions. The intent is to stabilize prices. This is not what happened here.

What happened here is a broker (Robinhood) decided to not accept certain kinds of orders. It wasn't a market shutdown, it was a specific manipulation to stop the purchase but _not_ the sale of an instrument, causing the price to drop.

I was referring to what happened yesterday by NYSE, not what Robinhood did today.

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