Apparently Robin Hood has to raise collateral, i guess this is because they support options which use leverage? Does this mean they operate on a fractional reserve?
There is always a lag time between when a brokerage accepts a trade, and that trade is actually executed. The brokerage puts up the money in the mean time. This is my guess.
I think it takes 2 days for money to arrive, but they let ppl trade right away. The T+2 thing always made me a bit nervous trading stonks in volatile markets.