The WSJ article on the topic has a little more specific information:
Japan’s SoftBank, which bought options tied to around $50 billion worth of individual tech stocks.
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Regulatory filings show SoftBank bought nearly $4 billion of shares in tech giants such as Amazon.com Inc., Microsoft Corp. and Netflix Inc. this spring, plus a stake in Tesla. Not included in those disclosures is the massive options trade, which is built to pay off if the stock market rises to a certain level and then lock in gains, the people said.
SoftBank bought a roughly equal amount of call options tied to the underlying shares it bought, as well as on other names, according to people familiar with the matter. It also sold call options at far higher prices. This allows SoftBank to profit from a near-term run-up in stocks and then reap those profits by unloading its position to willing counterparties.
So they are buying stock in tech companies, then compounding the bet that the stock price will go up by buying an equal dollar amount (in this case, corresponding to 11x the number of shares) of Out-of-The-Money calls. Then they wait for the price of the stock to go up, fueled in part by the Market Makers need to buy stock to hedge their exposure to the calls that SoftBank bought.
Once it's gone up, they then sell (presumably In-The-Money but at a greater strike price than they paid?) calls for more than they paid for the initial OTM calls. When those calls are excercised (because they are ITM) they sell the stock for a profit (price increase plus the premium of the covered call). Because they are buying large enough amounts of calls to cause the Market Makers to buy enough to increase the price of the stock, their strategy is more likely to work than it would be for normal investor.
One big unanswered question: do they sell their OTM calls before expiration after they increase in value? Or are they able to exercise them once they are ITM? I'd guess their goal is to exercise, since that helps them put more pressure on the MM's to hedge and while also letting them continue the strategy.
The reason this question matters is it affects how much stock is going to flood the market once SoftBank stops buying calls. SoftBank will presumably sell the relatively small number of billions of dollars of tech stocks they are holding at the right time, since they know when they'll stop buying calls.
Once the number of outstanding calls drops, then next thing that will happen is that the MM's will presumably sell their enormous hedge, causing the prices to drop rapidly. But obviously, the MM's don't want the price to drop until they have completed their sell off. Are they allowed to use the same strategy that SoftBank is using? That is, can they pre-sell a sufficient amount of (possibly discounted) ITM calls so that they can unload their hedge without affecting the price they receive? Or do they sell them on the market? Is there information available about what the MM's are holding as hedges?