It's basically bunnyhopping in the air. The key is to never press the forward key, only strafing, jumping, and moving your mouse side to side. Just look at the bottom left animation on this page and replace the middle mouse for your jump button.
http://fidosrevenge.com/bhop/
What I don't understand, is how people jump, look down while spinning 10 times and suddenly launch themselves 150mph towards someones face.