First, let's define a margin call. This is a demand from a broker to an investor to deposit additional money into a margin account when securities that were bought on margin have fallen in value past a certain point.
We send out a margin call by email when buying power goes below zero. (We talk about how margin buying power is calculated here.) This happens because the securities that you have bought with borrowed money have fallen in value.
Of course, because this is a simulation you don't have the ability to deposit more money into your account. So, in order to get out of a margin call, you must sell off some of your securities (preferably short sales).
If you don't bring your buying power above zero within two days, we will automatically liquidate part of your portfolio and make buying power positive for you. For more on this, see our definition on forced selling.