Gary Gensler, SEC chairman, has initiated a review of payment for the flow of orders.Robinhood Markets Inc. is on a collision course with regulators over a controversial practice that generates most of its revenue, as online brokerage gears up for a highly anticipated initial public offering.In its IPO filing, released on Thursday, Robinhood disclosed that 81% of its first-quarter earnings came from sending its clients’ stock, options, and cryptocurrency orders to high-speed trading firms-a practice known as payment for order flow.Payment for order flow critics-including the country’s top market regulator, Securities and Exchange Commission Chairman Gary Gensler-are wary of the practice. They argue that it poses a conflict of interest for brokerages, due to the fact that the brokers can either collect more money for selling their clients’ order flow or pass that money on to clients in the form of price savings“The entire business model of some brokers is in the crosshairs,” stated Tyler Gellsasch, executive director of Healthy Markets Association, an investor trade group.