This arrangement allows BlackRock to borrow Bitcoin, including for short-selling, without proving a 1:1 holding ratio.

These transactions settle in 2 to 30 days, all in cash, not Bitcoin. Off-chain trades occur outside the blockchain, remaining invisible on the public ledger.

https://www.altcoinbuzz.io/cryptocurrency-news/blackrocks-secretive-bitcoin-transactions-with-coinbase/