[kristjansson]

It's the other way around? You borrow substantially fewer dollars than the value of the shares you pledged as collateral so that the bank doesn't call the loan if (when) the value of the shares changes.

[asciimov]

For you and me, you are right, but those on the top get to play by different rules.

Also, while it's called a loan, think of it more like a credit line.