Your account starts out with $100,000. You are allowed to borrow an additional $100,000, giving you total buying power of $200,000. The additional $100,000 is a loan and is only given when you buy stock, and you pay it back by selling stock. The loan amount does not change the value of your portfolio except for the interest that is charged on the loan is taken out of your account. But if you borrow money to buy stock and that stock earns money, when you sell the stock you pay back the amount borrowed and keep the profits. The same if you borrow to buy stock and the stock goes down, when you sell the stock you pay back the amount borrowed and the loss comes out of your portfolio value. Hope this helps!!