Everyone has seen this trick. A juggler makes a High throw, then quickly bows forward catching the ball on their neck. Most people try to copy this feat, throw up a ball, lean forward & get struck on the head. Here's how it's done. But first...
Health warning! The human body was not really built for neck catches. The head is a very heavy part of the body & flicking it around puts a lot of strain on the neck. Don't practise this trick for more than a couple of minutes at a time. Not being able to lift your head without the aid of your hands is not nice. As anyone who has over done it to Metallica or Slayer will testify.
Also if you wear glasses take them off if you can or make sure that they are secure on your head. This trick involves fast head movement which may cause them to fly off & get damaged.
Just practise with one ball. Throw it straight up in front of you from the centre of your body. Don't throw forwards or behind you, just straight up. Don't throw too high for now either, throw the ball higher & it will come down with more force & will be harder to catch. Look straight up at the ball & watch it until the last possible second. As it falls to about an inch above your head, quickly duck your head forward & down, at the same time start leaning forward to absorb the force of the descending ball. Aim for the ball to hit the back of your head at the soft part just above the neck rather than the bony part of the skull, which would cause the ball to bounce away. When you feel the ball touch the back of your head, tilt your head backwards & hunch your shoulders up. This & the stooping forward of your body creates a pocket for the ball to nestle into. The ball should roll down the back of your head & into the pocket.
If it rolls down your back bend over further, if it rolls off the side of your neck hunch your shoulders up & back more & if it rolls back over your head then tilt it back further.
Once the ball has been caught on the back of the neck many people simply stand up & expect the ball to return to their hand, which unfortunately doesn't happen.
To flick the ball back to the hand with your head, first duck your head down to start the ball rolling forward, don't tilt your head too far forward or the ball will roll too fast. Keep your shoulders hunched up to make a straight channel for the ball to roll down, if you drop them it will roll off at an angle. When you feel the ball roll up onto the bony part of the back of your skull, flick your head backwards & straighten up your body. This will throw the ball straight up in the air in front of you for an easy catch. The ball will come off of your head with a lot of spin so when you go to catch it make sure you grab hold of it well or it may roll out of your hand.
If the ball flies off behind you then you straightened up too early &/or too hard. If it flies off in front of you then you flicked too late.
Remember not to practise this trick too much at once, it does put a lot of strain on your neck & you could injure yourself.
Alternatively you can go the other way. Don't duck your head down, keep your shoulders hunched up & slowly stand up straight. The ball will gently roll down the channel made by your spine for a simple Behind the back catch. Just put one of your hands at the base of your spine with your fingers outstretched & the ball will fall into it.