I'm going to agree with Bob on this one. The momentum of the board keeps it heading in the same trajectory as the skater. It generally doesn't last very long, considering the difference of weight and air resistance between the two of them, but it's enough for them to release the board and expect it to still be below them when they land. The more familiar the skater is with the tricks, the easier it is to predict where the board will be. Similar to how a juggler instinctually learns how long a ball will be in the air before he has to catch it again.