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What are EFPs?
An EFP is a spread consisting of 100 shares short stock, against a long single stock future (SSF) for the same underlying stock.
If you short an EFP (long stock, short SSF), then the proceeds from the short SSF will be greater than the purchase price of the stock, by an amount roughly corresponding to interbank market interest rates, after you take into account dividends expected to be paid by the stock. The spread will gradually decay toward zero by the time the SSF expires, at which time the SSF will cause your stock position to be sold, leaving you flat, and with a little extra cash earned as the equivalent of interest on your cash.
A short EFP position allows one to earn the rough equivalent of interbank interest rates on the entire account balance, which is more than IB or any other broker will pay on cash balances.
IB makes very tight markets on EFPs on IB's IBEFP electronic execution facility. It is essentially an ECN. IB customers can both take and add liquidity on IBEFP. You can even make your own markets if you want. The spreads on IBEFP are far narrower than one would pay by separately legging into each side of an EFP's spread.
EFPs can also be used to reduce the cost of financing a long stock position, or to earn the equivalent of interest on the proceeds from a short stock sale (far more than any broker will pay), and they can also be used to increase leverage beyond that permitted for stock trading.