The U.S. Securities and Exchange Commission proposed rules this week that could threaten a key revenue source for popular smartphone trading applications like Robinhood and potentially change the way everyday Americans interact with the stock market. The best execution proposal could have a more direct impact on retail investors than the other three proposals up for consideration. That's because the language in the nearly 450-page proposal targets a practice known as payment for order flow, which has been credited as allowing brokers to offer commission-free trading by passing their customers' trades through a third party, from which the brokers collect a fee.