A throw that goes behind the back funnily enough. There are two main behind the back throws: this one (also called a back cross) & Over the shoulder. I've always thought that behind the back is a contradiction in terms & actually translates to 'in front of you' but everyone usually tells me I'm wrong & that's in general, not just juggling.
Throwing behind the back is different from any other throw covered so far in these tutorials in that you can't see what your hands are doing. In juggling seeing your hands is not important in the slightest. Juggling develops good spacial awareness & a high degree of coordination. You will be able to feel what you are doing; you will develop the ability to feel where a ball is going just by the way it leaves your hand.
Practise the throw with one ball to get the feel for it. Hold a ball in one hand & stand up straight with your shoulders back & your arms hung at your sides. A behind the back throw consists of two parts: the swing & the twist.
For the swing bring your forearm up & to the outside, keep your upper arm by your side but angled back so that your forearm will pass behind your body. Let your forearm swing down & behind your back, swing all the way until your hand is level with your opposite shoulder blade & release the ball. Without putting much force into the swing the throw should rise to above head height. Look up & to the side to spot the ball as it pops up over your shoulder. Don't worry about not seeing the ball leave your hand, this is known as a blind throw & occurs in hundreds of juggling tricks. To make the catch you only need to spot the ball as it peaks (for only 50 milliseconds, or for one inch of its flight path apparently).
The other part of the throw is the twist, you probably did it without realising when practising the swing but I'll talk about it just to make you aware. Put your arm behind the back with your hand at the point of release (your opposite shoulder blade). Now twist your wrist as if you were turning a door knob. You will twist so that your arm would roll upwards if it were touching your back. At the end of the swing you twist your wrist in this direction. It is the twist that makes the ball go forward when it is thrown so that it travels from behind to in front of you.
The throw should be made entirely with the arm, don't lean into or away from the throw, leaning will lessen the effect of the swing & will reduce the height of the throw. Lean back a little with the shoulders only.
Put the swing & the twist together, standing upright all the time. Practise until you can throw consistently from both sides. Remember to throw higher, when throwing behind the back in a cascade you will need the extra time to make up for the longer dwell time.
If you find yourself throwing balls into the back of your head it is because you are only swinging your arm as far as your spine. Concentrate on swinging all the way to your opposite shoulder blade.
Now try putting it into a Cascade. Make a normal cascade throw a little higher before throwing one behind the back to give you enough time to complete the swing. The behind the back throw will rise higher than your cascade & will affect the rhythm. Wait for the behind the back throw to fall before throwing the next cascade throw or your pattern will catch up with itself. Count out, "1, 2, 3, 4, 5, Back, wait, 1, 2..." remembering to throw the five higher than normal.
When moving up to one ball every time & so on, where you look becomes more important. For the same ball every time you will probably switch your head back & forth. For every right & every left, tilt your head back to look up over your opposite shoulder. Every ball behind the back is a 'must learn' skill. Look straight up above your head & move your eyes from left to right to spot each ball as it peaks.
Remember that you are throwing higher so the rhythm will be slower (I talked about this in the High Throws tutorial).
You will also need to stand with your feet over shoulder width apart & with your toes pointing diagonally outwards. This will give you a solid stance to cope with the constant swinging of your arms which will affect your balance. When you can do continuous back crosses while standing on one leg you can consider the trick mastered.