It's more forward momentum than altitude. Dive bomb, release the trigger, and glide alittle further, depending on your controls up or down on the stick will make you go higher. Using some of that forward movement to glide up. But not terribly high.
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1. Start your glide and pull back on stick so you are going level. 2. Still holding the stick back pull the dive button and start your dive. 3. Let go of the dive button, still holding the stick back and you will have increased speed and glide upwards now for a bit. 4. As your glide starts to slow down and level off, repeat.
Its really simple once you play around with it a bit. The most difficult part is figuring out how long to hold dive and release. Not long enough and you won't gain any speed or height. Too long and you can end up faceplanting into a street or water while you are leveling off :).