The back flip is one of the easiest swing dance aerials one can learn. There are many different names such as walk in and knickerbocker, but they all operate on the same principles. Learn the principles before you try it.
Principle one : Axis is everything!
Regardless of the entry (one footed or two), platform use (hand platform, shoulder protect platform, free flying tuck), or any other factors, somebody is executing a back flip, probably a back tuck. And back flips, at least the standard ones, are flipping around a single axis, the flier's shoulders. That needs to be level (parallel to the ground) and stay level throughout the flip.
Just to make the math easy, let's say a flier's shoulder is exactly five feet from the ground. And say the flier squats down exactly six inches when doing the flip. The rotational axis would be just about 4 feet six inches high, or, basically the entire flier's body less the head, goes completely around the 4'6'' axis.
It is mostly in hands of the leader to protect this axis. As the flier squats down, the leader then locks the axis in place using one of many possible platforms or connections. Depending on the strength of the individuals, some type of force is executed, and the rest of the flier's body goes around the axis as the leader protects it.
If you're watching it from the side, and if you imagined the flier without a head, you could then imagine almost being able to put a broom stick at about three inches behind the leader's arm at 4'6'' in height, and the flier would flip over it. (becasue the flier has a head though, the broom stick would get in the way, so I don't suggest that.!)
Principle Two : If you want height raise the axis while in flight.
I do this demo all the time and it's very effective. Using a simple knickerbocker grip (left hand across the shoulders of the flier acting as the axis protection arm, and right hand lower back or coccyx level depending on the skill and strength of the flier) I start a simple back flip. Right after take off, I simply take my left hand and pick up the flier's axis anywhere between 2 to 12 inches.
Be really careful here. Three inches of lift translates to about another foot of falling for the flier, at least it does for me. Stronger leads who master the technique can really get a lot of lift. It tends to freak out the flier, who then has to react to at least the amount of lift plus any extra upward inertia. (she goes higher, which means she has to further to fall. It's scary.)
Principle Three: Fliers protect your axis at all costs.
Back flips are flubbed all the time when fliers freak out. It makes sense. On one side there's a solid platform leader, and on the other side there's thin air. Anybody in their right mind is going to think 'grab something or else' but that's the worst thing a flier do. Keep your left hand along your axis. Extend your axis, point your arm and hand straight out parallel to the ground if you have to.
I like to teach holding a tray with your left hand, like a waiter does, palm up. I say imagine all the way through the flip that you have to hold a tray. And I suggest extending the tray a good bit away from you, just like the waiters do. If a flier maintains that 'parallel to the horizon' tray holding thing through the entire flip, you can almost bet the axis will be more level than if the flier does something crazy with her hands.
Also, there are several back flips where the flier is tightly connected using her right arm. That one single connection can totally protect at least the height of the axis on one side. I show that trick all the time in my classes.
Principle Four: Spotting is everything when a new flier is getting the hang of it.
Back flips are some of the easiest and safest things you can possibly learn if you have a trained spotter. Seriously, there is literally nothing that can go wrong if your spotter knows how to spot a back flip, regardless of entry. And, spotting all back flips is quite simple.
It doesn't make any sense not to do your first few back flips with a spotter there. The consequences can be fairly dire if a new flier and leader can't work it out, so for heaven's sake get a spotter.
On the other side of safety though is the flier's mental stability. Trust me, she's going to feel MUCH more comfortable with a spotter until she's gone over about a dozen times. Eventually spotting will turn from active, to passive, to none. It happens naturally though, don't push it. The last thing you want to do is 'challenge' a flier's mentally to do something she doesn't feel safe doing. Best bet, just spot until she tells you she doesn't need you.
Principle Five : Most back flips are simple and safe, but some are very dangerous.
Some back flips I could teach my 7 year old cousin in about 5 minutes. Not all are that way though. Some require a honed posting skills. Others require completely different connections and different body parts to provide lift. For heaven's sake don't try a new one or don't try to 'break in a new partner' without a spotter you trust. Well, I'd do better than that. I'd say 'don't try a new back flip without a spotter THAT I TRUST'.
I personally have never seen anybody injured via a back flip. I believe there are a two simple reasons for this.
Firstly, if I'm leading or following the flip, I know when to get a spotter. And on top of that, the spotters I personally work with have years of training and have proven to me time and time again that if they are next to me when I am doing something difficult, there will be no injury.
And secondly if it's me on the spotter side, I know exactly how to keep people safe, just like the spotters I mentioned above.
What I will say is on many occasions, as a spotter, and on several of the 'self spotting' back flips (those that can be effectively spotted by the leader if the leader also knows how to spot the aerial) I've snatched girls out of thin air and out of situations where I doubt they would have even lived. Yeah, nobody's ended up in a neck brace, but I have absolutely no doubt that it could happen. It only takes about one half a second to get extremely dangerous, and consequences can be dire on the more advanced back flips. So again, for heaven's sake, be safe. Get a spotter until you know exactly what you are doing. I work with spotters all the time in my practice sessions. It doesn't mean I suck, it means my fliers will live. Safety first!!!!!
Save a bathtub falling from the sky, or maybe a bruised foot, there is NO EXCUSE for injury on a back flip.