When a whale buys and sells, he has to do it a bit at a time over many days, or it has a huge effect on the price.
Yeah, some of us could drive it up or down 10% with a million buy or sell, but it would go right back to where it was afterward, and we would be left looking silly having paid a 5-10% premium. Look at all the times recently it has spiked one way or the other and then come most of the way back right away. The real whale buys or sells 1M over and over again over the course of days, and causes an uptrend or downtrend.