You are right. For example, you short NT 1000 shares, the price is $12.00. if you open a cash ccount, you should have $12000 cash in your count, then you can short it. after three days, now the NT price is $11.00, If you cover at this price, you earn 1000 dollars. Anyway, if you don''t want to cover it, No problem. But if the price is going up, now NT is $13.00, so you lose $1000 dollars, if you don't cover it , you should deposit another 1000 dollars in your count and make it banlance. So simple. It is not easy to do it, perhaps you never know the market trend. Up and down same opportunity. Good luck!