The barrel roll, or "el rollo" is one of the most classic and fundamental moves both in contest and free surfing. You can do a roll on almost any wave. The rolling technique is very versatile because, unlike most aerial maneuvers, it requires little speed. The crucial component is the lip of the wave and the force you exert on it.
The outcome of this maneuver depends on your approach to hitting the wave's lip. You must properly contact and hit the lip in order to project yourself in the air and start rotating the roll.
Drawing down the line: After bottom turning, you want to hold your line to gather speed and simultaneously look down the line in front of you to identify the best part of the lip of the wave to execute your roll. Speed is important but not a requirement. After getting up to the lip, you let the wave and gravity pull you down as you complete the rotation.
Focus on the oncoming lip and how the wave is shaping and shifting (in reference to the wave lip and riding down the line). Proper set up and timing of the approach will help you successfully accomplish the rest of the maneuver.
Hitting the lip: Now you are approaching the lip. There are basically two types of lips that you will encounter. The first is an open face lip like in this sequence here of Andre Both. In an open face lip you will land and continue riding on the wave. The other type of lip is a “closeout.” This term describes the wave’s lateral wall closure
For an open face roll, keep your board parallel with the lip and wave to continue riding on the face of the wave when you land. When you hit the lip, you are gliding with it and letting it carry you into your rotation. After landing you will have successfully completed the trick!
For a closeout roll, you should keep your board perpendicular to the wave and lip in order to land facing the sand in the whitewater. You are projecting from the lip, [up and out] (more so than an open face roll).
In ether case, the rest of this information will apply for both instances:
The best part of the lip, as you can see in Image 2 of professional bodyboarder Andre Botha, is where the wave becomes vertical and pitches over.
As you are riding on the way to the wave's lip, you should be leaning forward on your board to gather speed. Bottom turn to the lip by leaning your weight towards the wave on your inner forearm. This will help you go upwards towards the lip.
As you hit the lip, you should shift your stomach back on your board and simultaneously arch your head back, which are techniques in the next "Rotation" section.
Begin the rotation by leaning your head back. Keep the board close to your stomach. Use your arms and board to lead the way, followed by your head/torso/back. Be as smooth and fluid as possible.
Don’t worry about legs, they will follow with the rest of your body.
This helps the rotation and if swimming competitively, you will receive good style points!
You need to get pitched over the wave to rotate and land properly whether you force it or the wave's lip pitches over enough for you to go with it.