No one can predict the future. That is true! But the big boys can predict the direction (up or down) when they trade with large of volumes. Not the future price exactly. How? They can see the DOM (Depth of Market) - they can work out from there.
For example on BUY EUR/USD Depth (Ask and volume columns):
Let's say one trader sees this DOM. He can afford to buy 80K volumes and he knows the price will jump from 1.13000 to 1.14025 (up to 1.14000) -> that is 100 pips. My point is that he can predict the direction that this ASK price will go up and the spread might gets wider (high volatility).