This from David Moody:

Go to the red base. You can do this at either base, but red base I find
is a bit easier. Go to the rear of the base, where the Banshee flies
out. Go up the left side of the double ramp. Look to the left
on the base and there should be a small hole between the wall of the
base and one of the pillars on the base. Look a bit above the
hold and jump. When you get clsoe to the hole, crouch and try to get
stuck in the hole. Once stuck, there is a small brown box (its the
inside of one of the things that you are in, right in front of you, it
looks like a brown box). Look at that and hold foward. Tap A and you
should hit the ground and fly up. For a double jump, aim into the
telport and you go through and bounce again.