This came up a year or two ago on the rec.skydiving ng. Here is a cut and paste of the post made by John LeBlanc in reply to the thread. I hope it answers your questions. FYI, when I do them, I start with double front risers or a carving front riser turn to build up speed. Nothing radical, a 90 degree turn will do the trick. Then let the canopy plane out on its' own with neutral controls. With an efficient canopy and the extra speed, it will start to climb a little. At this point, I gently go to about 1/4 brakes to increase the climb and angle of attack. Be careful to be smooth and do not let too much speed bleed off. While the climbing canopy still has some of the extra speed induced by the front riser(s), I smoothly ease up a little on one toggle, while at the same time smoothly adding a little more of the opposite toggle. As you go over the top, begin to smoothly return to neutral and depending on your skill and the canopy, you may have to counter steer at the bottom to stop it on heading. I find it helpful, at the beginning, to pick a spot on the horizon for a reference to start and end the manuever. Your canopy should rotate about this point as you fly towards it. Imagine spiralling down around a point on the ground, then tilt that axis up 90 degrees to the horizon. You are going to have to be capable of flying a canopy at a higher wing loading of about 1.5 or higher.
"This barrelroll talk is getting pretty wild, and I think someone could get hurt.
First, this stuff started years ago when people saw the barrel roll I made (in front of cameraman Gus Wing) that was used in the Tom Sanders movie "Over the Edge." It is in the PD commercial at the end, if you're still watching by then. Its not in the Blockbuster movie rental version.
Second, it is not a tandem photographers trick, and it is not bull****. As to how far above the canopy you can get, Tom freezes a frame at the top with the horizon in view, so it is pretty clear how far over the top it is. (I did buy beer, but I doubt I was the first.) At the same time, in the upside down part of the maneuver, you are starting to go "downhill," so the canopy is slightly behind you. It needs to be there to keep the canopy loaded up, for self-preservation. This doesn't mean it is not a barrelroll. People doing barrelrolls in aircraft not certified for aerobatics do the same thing: They keep the wing positively loaded all the way around, and they're going downhill at the top. A roll made in a straight line is called an aileron roll, and parachutes won't do one. If the nose gets too far out in front of you, the canopy will get a negative angle of attack, and you will fall into it, whether the canopy is airlocked or not.
Third, and most important, it is not a radical maneuver! Don't start yanking down on the toggles and risers aggressively, or you'll be in real serious trouble. It is an energy conservation maneuver, where speed is transferred into lift for a steep climb, then a smooth, gentle "turn" is made in brakes, which creates the barrell roll. One toggle is only three or four inches lower than the other. It is very similar to a big wide spiral dive at the ground, but it is made horizontal to the ground instead.
If this is not clear, don't mess with it. I've been getting questions on this for years, and I've always kept quiet on the newsgroup about it until now, but the guesses and poor advice I'm seeing is really troublesome. I've talked to many people about this, and many of them have been in deep... trouble. Certainly, any manuever involving stalling one side of the canopy, either on toggle or rear risers, is a real mess. Also avoid any radical turn reversals from right to left or visa versa, unless you like spinning line twisted mals. You need a really efficient canopy, but not one that is too small, as this kills lots of efficiency. The elliptical planform helps keep the turn rate up at the comparatively slow speed over the top. The PD Stiletto, and Vengeance do nice barrelrolls. I haven't flown a Diablo yet, so I couldn't comment on it.
So if you want to risk it, start gradually. Always stay smooth and slow, not radical. Start with a clear and pull alone at 13500, and stop experimenting at 5 or 6 thousand feet. Work gradually up to it over many, many jumps. It is an energy conservation maneuver, not an aggressive testosterone thing. You might want to insure your rig is good for cutting away from line twists with no problem. Large three rings and hard channels in the risers for the end of the cutaway cables are prudent, as are hard cutaway cable housings. Fly smart, don't kill yourself. As Jacques said, "Please don't try this based on my thoughts."