This is another blind catch made by the hand behind your head.
Start with one ball; stand up straight with your head up. Hold the ball with your forearm parallel with the ground & pointing diagonally out to the side & in front of you. With your other hand touch your hip. Turn your head slightly to the throwing side & move your eyes to look at the ball. Throw the ball backwards over the same shoulder so that it peaks behind your head. AFTER you have released the ball, lift your other hand from your hip & reach behind your head to make the catch. Placing your catching hand behind your head & throwing into it is far too easy. To make a behind the head catch in a three ball pattern you will need to be able to lift your hand into the catching position after the ball has been thrown which is why touching your hip is good practise.
Just like in the Behind the back catch & with all blind catches, spread the fingers of your catching hand out as wide as possible & close them up slightly to make the 'dead spider' shape, this reduces the chance of the ball bouncing away when it hits your hand. Your fingers act as a cage to keep it in place.
Practise throwing & catching behind the head from side to side, remembering to place your catching hand by your side before each throw. You will probably find yourself ducking your head forwards to make catches, try to avoid this by throwing further backwards. It may help to bring your throwing arm inwards & throw from in front of you more than to the side.
To put the behind the head catch into a Cascade you'll need to learn another throw which is made by the catching hand. It is certainly possible to use a normal cascade throw to empty the catching hand before you make the behind the head catch, but you won't have much time to whip your hand behind your neck ready for the catch. A slightly slower & more controlled way of doing it is to make the throw & move your hand behind your head in one motion.
Take one ball & swing your arm with your elbow bent using outside scoop. When your hand is at shoulder height & your palm is pointing to the opposite side release the ball gently, DON'T THROW IT, under the momentum of the swing it will rise, peak & fall just like a normal throw. There is no need to add any extra force. After you have released the ball your hand is up higher than normal & has less distance to travel, just continue the swing until your hand is in position behind your head.
Move up to two balls & practise the behind the head catch with the combined carry & throw move. Start with a ball in each hand, first throw to behind the head & then swing, release & make the catch with the other hand. The two balls swap hands during this move. If you practise with two different coloured balls, always throw the same ball behind the head first, this way you'll learn the trick on both sides evenly. Practise this exercise until you can do it from both sides without dropping, bobbing your head or moving your feet.
Take three balls & juggle a cascade. Pick a ball, catch it, move your arm into the throwing position & throw it behind your head. Concentrate on the behind the head ball, but follow it with your eyes, don't turn your whole head or you will lose sight of the other two balls. They should remain in your peripheral vision at all times. Practise picking a ball & catching it behind your head again until you can do it from both sides without dropping, bobbing your head or moving your feet.
When you can do that choose a different coloured ball from the rest & practise making every catch with that ball behind the head. Keep your head facing front throughout & just move your eyes from side to side. Juggle with your shoulders as far back as is comfortable, this reduces the amount of movement needed for the swing & makes it a little easier on the arms. Also with the other two balls hold on to them for as long as possible, increasing the dwell time slows the pattern down & gives you more time to watch the paths of the balls.