It’s not banks but I think the GP is trying to describe a margin loan. Basically you give (say) an exchange $100, ask them to buy $1000 worth of stuff using your $100 as margin, and if the stuff you bought looks like it is going to be worth less than $900 (ie if they would make a loss by selling all your stuff) they will call you and ask for more collateral or, if you are too slow, sell all your stuff for eg $925, leaving you with a less of $75.