Quote from kashirin:
I put around 150K. After I sold those EFPs my balance was less than 1K
All 3 stocks soared and after some time my balance went down to -10K (I haven't traded at all)
I don't really know why but probably this is how SSFs work
So by some way I'm paying interest for those 10K.
Suppose you hold 150K of stock, but NOT in the form of EFPs. Suppose you bought it with 150K of cash, so that you didn't have to rely on the broker to extend you a margin loan. If the stock goes up, then so does your account equity. If the stock goes up, your equity goes up with it, so that the increase will not generate a negative cash balance, and you will not need to rely on a margin loan, and you will not need to pay interest simply because your stock increased.
Now suppose, instead, that you are holding the stock as part of an EFP. EFP stock is hedged by the EFP's futures leg. If the stock goes down, you lose virtually nothing, because the short futures increase in value by almost the same amount. If the stock goes up, you gain nothing, because the short futures go down in value by almost the same amount.
If your EFP stock goes up to $160K, but your account equity remains the same at $150K, then the broker must extend you a margin loan of $10K, so that you can continue to hold $160K worth of stock in an account having equity of only $150K. This is the source of your negative cash balance of -$10K. The solution, when this happens, is simply to readjust your EFPs so that your EFP stocks will maintain value close to your account equity. This is a different way of saying that you are trying to maintain a cash balance close to zero.
If your EFP stocks go down, let's say, to a value of $140K, but your account equity remains unchanged at $150K, then you have a positive cash balance of $10K. This $10K should be put to work earning interest, but instead it lies there doing nothing. If this happens, then you should re-adjust by selling an EFP to put this extra $10K to work; otherwise, you miss out on the interest you should be getting.
Note that increases and decreases in the value of your EFP stock leg will affect your cash balance in a way that the EFP futures leg will not. This is a very important asymmetry in how the two legs affect your cash balance.
You want to avoid cash balances that get too far above zero, as well as those that get too far below zero. Another way of saying this is that you want to avoid holding EFP stocks that have value too far above, or too far below, the amount of cash you have reserved for EFP investing.
I hope this explains what happened to your cash balance as a result of fluctuations in the value of your EFP stocks.