A flyer may choose to 'fly high' during his turn instead of making a normal flying move. This represents a flyer ascending far into the air above the battlefield. All that observers on the ground can see is a tiny black dot amongst the clouds. These rules represent models flying high, diving down onto the battlefield, and engaging in combat against each other above the clouds.
By necessity the rules are rather long, but large flying creatures are rare and special, so it's worth the extra effort involved! However, we would recommend that players ignore these special rules until they have learned and used the rest of the flying rules. Some players prefer to ignore the Flying High rules altogether, and if you and your opponent are so minded there is no reason why you shouldn't do so - simply assume that high winds make it impossible to fly high.
At the start of his movement phase, when charges are declared, the player also declares which of his flying models will fly up high. These models are removed from the tabletop during the movement phase. Any models which fly up high are placed on a side table, or some other convenient spot designated for all models from either side which are up high. The model is now said to be 'high' or 'up high'.
A flying model that is already up high at the start of its turn may either remain where it is or dive down onto a designated point on the battlefield (but not into a wood!).