Even if it was worth near zero (market price between strikes, very close to expiry), Robinhood holds collateral for the difference. ie, if I sell a $50P and buy

Even if it was worth near zero (market price between strikes, very close to expiry), Robinhood holds collateral for the difference. ie, if I sell a $50P and buy a $45P, Robinhood holds $500 cash collateral to cover the difference. The maximum you can lose, once thing are settled, is the difference between the strikes less the premium collected. With a put spread, if I sell $50P and buy $45P, the holder of the $50P I sold can exercise at any point, and I have to buy his 100 shares at $50/ea. Let's say I got a $2.50 premium ($250 total) on that spread. At that point, my account is -$4750, +1 $45P. Perhaps the premium on the $45P is $0.50, so my account's nominal value is -$4700, +100 shares at this point. However, my $45P expires in 3 days, so maybe I want to hold on and see if the shares I had to buy at $50 will increase to $55, my $45P expires worthless, and I can then sell them for $5500 total, leaving my account +$800. Or, worst case in the other direction, the price drops below $45, and I exercise my $45P to sell them for $4500, leaving my account -$250. But until expiry or assignment, my account's nominal value is in the red.