This is a trick where one ball is caught between the chin/face & the shoulder creating a nice pause for applause in a routine. When done well this move can momentarily trick an audience into believing that the ball has stopped in mid air.
If you are using solid balls you might like to learn this trick trapping the ball with your cheek which is a lot softer & less painful than jamming the ball with your jaw.
Take one ball & make a short soft lob towards one shoulder, aim for it to peak just before it touches you. As soon as it does peak, lift your shoulder & tilt your head so that the ball brushes between your cheek & shoulder & falls back into your hand, don't worry about trying to trap it there for now. The throw is all important, if the ball is thrown too hard it will be tricky & perhaps painful to trap. If it is thrown too softly you will have to lunge forward to make the catch. Practise getting the throw right, just keep tossing one ball up to each shoulder & hunching your shoulder up to your cheek, just work on making consistent throws that peak at the right place.
Once you've perfected the throw you'll find the catch quite simple. The main problem is that the chin & the shoulder are both very bony areas of the body. You will probably have difficulty with your balls bouncing off the corner of your shoulder. If you are using soft bean bags it is not as big a problem than if you are using solid balls. It is more reliable to make a catch between two soft muscly parts because the muscle will absorb the shock of the ball & it will also mould itself around the ball as you squeeze to get a better grip.
Put your hand on your shoulder, wiggle your arm around & have a good feel of what your shoulder is like. No seriously, get to know your dimensions. Everyone has slightly different proportions & bone structure but we are all pretty similar. This is what I feel. With my arm by my side I feel a hard, narrow bony ridge sticking out on the end of my shoulder, I try to avoid catching the ball at this point. As I slide my hand in towards my neck, the bone disappears & there is a muscle going form my neck to my shoulder. It is quite steep & narrow & slopes out to the side, but if I lean away to the opposite side a little (to the left when catching on my right shoulder) & if I move my shoulder forward it flattens out into a wider platform. It is to this point that I aim my throws. Wiggle your shoulder, stick your arm out in different directions & move about until you find a position where your shoulder has a soft, wide & flat area to catch a ball on.
Do the same with your face too, it is much less complex but worth examining. Touching my face the first thing I notice is that I haven't shaved today. Feel along your jawline so that you know exactly where it is, the jaw bone is very sharp & has a very pointy corner at the end so avoid using these parts to trap the ball. You can use the area directly under your chin, in which case you'll need to turn your head round as far as you can. Personally I prefer to use my cheek. I push my shoulder up & lean my head to the side so that my jawbone is out of the way. After successfully catching a ball in a shoulder trap the ball sits on my shoulder like a parrot on the soft platform close to my neck & it rests against my cheek. There is no need to squeeze really hard, if you need to squeeze to hold the ball in place try adjusting your shoulder position to find a point where it will sit more comfortably without applying much pressure, then aim to catch in that position.
Try making a shoulder trap from Under the arm & Under the leg or from a Body bounce. Traps from Behind the back & Over the shoulder throws are very impressive & also very possible.