Not to try and sound all cool and hardcore but backflips are a 100% 'balls' trick.
Obviously more balls are required to do it in FMX and even more still for the double, but all backflips consist of is leaning back and holding on.
All you need is a foam pit to practice, no respect for your life or body and massive testicles. Then you're good to go.
Have a search on YouTube for the guy mentioned in the article who died crashing a backflip variation in Costa Rica(?).
Stephen Murray, a professional English BMX rider, snapped his neck crashing a double backflip. He is paralysed from the neck down now.
This all brings me to my point, there is pushing the sport through skill and then there is pushing it through disregard for your life. After watching a guy I grew up looking up to when I was a teenager crash onto his neck, ruining his life and devastating his wife, I can no longer get excited about crazy big stuff like double backflips.