The short answer is "nothing". The retail FX market is purely a speculative market. No physical exchange of currencies ever takes place. All trades exist simply as computer entries and are netted out depending on market price. For dollar-denominated accounts, all profits or losses are calculated in dollars and recorded as such on the trader's account.
The primary reason the FX market exists is to facilitate the exchange of one currency into another for multinational corporations that need to trade currencies continually (for example, for payroll, payment for costs of goods and services from foreign vendors, and merger and acquisition activity). However, these day-to-day corporate needs comprise only about 20% of the market volume. Fully 80% of trades in the currency market are speculative in nature, put on by large financial institutions, multibillion dollar hedge funds and even individuals who want to express their opinions on the economic and geopolitical events of the day.
Because currencies always trade in pairs, when a trader makes a trade he or she is always long one currency and short the other. For example, if a trader sells one standard lot (equivalent to 100,000 units) of EUR/USD, she would, in essence, have exchanged euros for dollars and would now be "short" euros and "long" dollars. To better understand this dynamic, let's use a concrete example. If you went into an electronics store and purchased a computer for $1,000, what would you be doing? You would be exchanging your dollars for a computer. You would basically be "short" $1,000 and "long" one computer. The store would be "long" $1,000 but now "short" one computer in its inventory. The exact same principle applies to the FX market, except that no physical exchange takes place. While all transactions are simply computer entries, the consequences are no less real.