However it’s not an acro manouvre, but pilots who want to fly acro must learn it first.
The Deep Spiral is actually the most effective height-loosing manouvre. During the Spiral the pilot is about at the same level with the glider, while the leading edge is facing to the ground (even in line with the horizon), and the pilot turns very fast around a center near to the canopy. The sinking rate could be more than 20m/s and the pilot’s rotation speed 110 km/h which also generate a significant G-Force.
Shift your weight to the right and start to pull down the right brake gradually. The glider start to turn and accelerate into a Spiral. If you pull more brake, the leading edge starts to be facing to the ground and the descent rate increases like hell…
YOU HAVE TO LEARN THIS MANOUVRE PROGRESSIVELY! AT FIRST START PRACTIECING ONLY BY DOING STEEP TURNS WITH LESS WEIGHT SHIFTING AND BRAKE INPUT, TO LEARN SAFE HOW TO LEAD IT OUT.
Due to the pretty high cross stability of the paraglider construction, if you center yourself and release the brake, usually the glider exit from the Spiral spontaneously in few turns. You can also help it by pulling the outer (left in our case) brake.
Many of the gliders, especially with low aspect ratio (DHV 1-1-2) are „spirally unstable”! It means if you release the brake in a fast Deep Spiral (let’s say over 14 m/s) it accelerates more, instead of exit from the spiral, and it doesn’t recover for itself if the pilot doesn’t do anything! It’s called „Nose Down” or „Over The Nose” Spiral. If it happens, pull down the outer brake really hard (perhaps with both hands!), or pump both brakes symmetrically as hard as you can, to get out the glider from the stable position.
The absolutely biggest danger of the Deep Spiral is if you can’t lead it out and you approach to the ground at very high speed (possibly with more than 70 km/h!). IN THIS CASE, IF YOU’RE CLOSER THAN 300 METERS TO THE GROUND, IMMEDIATELY THROW YOUR RESERVE, OTHERWISE YOU WILL PROBABLY DIE, EVEN IF YOU CRASH INTO WATER!!! In a Nose Down Spiral the pilot has to stand up against high G-Forces (possibly over 4 G!) and because of the permanent loading, the blood goes out from the head and it can induce loosing the vision and furthermore black-out, what’s also definitely equal with death!
The DHV test the gliders only up to 14m/s in Deep Spiral, so over this descent rate, the behaviour of the glider can be very different!
Please read also Bruce Goldsmiths’s article about this topic
Anyway, you can also get very big collapses if you lead out the spiral too fast, without bleeding out the energy gradually and you don’t control the situation right.
Be very careful and learn this manouvre gradually!