They are the Market Makers, they create and assign all the fills, both public and institutional trades. The market makers ARE HUGE INSTITUTIONS, (ARCA, CDEL, etc) - - -they have ALL the power. All public orders have to pass through market makers hands. Every transaction on the entire exchange is underwritten by market makers. There is no public to public trading, the MMs broker all fills and get a fee for doing it (SEC transaction fee). MMs are "middle men" on every single trade, bar none.
To make sure their lateral trade goes through uninterrupted, they simply sequence it to occur in that order in the fill sequence. It happens in tenths of a second, no one notices.
To make sure their lateral trade goes through uninterrupted, they simply sequence it to occur in that order in the fill sequence. It happens in tenths of a second, no one notices.