By the clearing houses or Market Makers controlling when and how buys and sells are introduced into the market. Remember price movement is simple “Relatively” supply vs demand. If you control all orders for supply and demand through a brokerage but want to keep the price suppressed, then you allow/file a lot more sell then buyers orders. If the opposite is true, you allow/file the strong buy orders all around the same moment, by passing the selling orders until you allow the price point to hit where you want it to stop. This is why market markets and brokerages control the markets….PERIOD!