It was once explained like this to me. Sometimes a heavy hitter falls asleep after a big lunch. The closing bell wakes him up, and since he's a heavy hitter, he hits heavy. He runs up to the spec at about 4:01 and does his business, which then prints.
Sorry, but it was explained wrong. No person is going to make a trade if they are not at the post at 4:00.
Back when I was trading WMT, I saw 1 mil plus blocks go off on time and sales when it was trading at near 50.