IB's site explains it in plain English, at the link given. What's unclear about it?
https://www.interactivebrokers.com/en/index.php?f=27462
By joining the Insured Bank Deposit Sweep Program, eligible IB clients can obtain up to $2,500,000 of FDIC insurance in addition to the existing $250,000 SIPC coverage for total coverage of $2,750,000. Clients continue earning the same competitive interest rates currently applied to cash held in IB accounts. IB sweeps each participating client's free credit balances daily to one or more banks, up to $246,500 per bank, allowing for the accrual of interest and keeping within the FDIC protected threshold. Cash balances above $2,750,000 remain subject to safeguarding under the SEC's Customer Protection Rule 15c3-3, backed by the firm's equity capital, which exceeds $6 billion.