Foundations: Sit to Sit Backflip
The four stages of a backflip from head-up, by Axis Flight School...
Why is a bungee on your leg straps part of making a rig free fly friendly? I present to you – how to fall the funk out of your rig without loosening your straps.
The reason for the tie is demonstrated in this video. Without it, you could fall out of your harness without loosening any straps. Having it may help reduce that risk significantly.
In a sit, your leg straps can ride up near your knees. This creates a hole you can fall through if there is an upward force on your rig. This type of force could be caused by a premature deployment at an inopportune time.
In the video the rig is put on just as I would to jump it. All straps were 'jump tight'. The issue isn't how tight the rig is, it is the hole created when your leg straps ride up. The bungee or pull-up cord that ties the leg straps together helps prevent them from being able to separate and work up to the knees.
Even with the bungee it may still be possible in some situations for them to shift. Just be aware – don't let your leg straps shift towards your knees.
The super-scary video of grandma almost falling out of her harness was caused by a similar issue. The way she squatted and pushed down and back in the door allowed her to fall through the same hole. Tandem harnesses are supposed to have a modification that keeps this from happening in that situation.
Generally you need gear that stays buttoned up at high speed and different angles to the wind. Flaps, covers, bridles and pilot chutes need to be designed to stay shut and in place. Most newer gear is FF-friendly. Older gear may not do as well, as it may have been designed for belly speeds & attitudes to the wind. Also, keeping your gear in good condition is important for FF friendly (tight pilot chute pouches etc.). If in doubt, ask an instructor or FF coach to check your equipment.