Just as you can throw Under either leg from either hand, you can also throw over either shoulder from either hand. Despite both types of throw both going behind the back & over a shoulder, throwing over the opposite shoulder is called a Behind the back throw or a back cross & throwing over the same shoulder is just throwing over the shoulder. Hmm.
An over the shoulder toss involves your entire arm & a little body movement too. Before practising the throw stretch your arm & especially your shoulder muscles for five minutes, the motion can be quite awkward & unnatural at first & I don't want you to pull a muscle.
Hold one ball with your upper arm by your side & your forearm pointing in front of you as if you have just made a catch in a cascade. From here let your hand drop without moving your upper arm & swing your forearm down in line with your feet allowing your wrist to turn in towards your body along the way. When your hand is level with your leg your palm should be facing your leg. At this point start to move your upper arm outwards so that your elbow points to the side, this will allow you to swing your arm behind your body & twist your wrist so that your knuckles point directly backwards. Your forearm should point mostly to the opposite side but angled slightly back, fold your wrist forward, hunch your shoulder & lift your elbow as far as you can before you release the ball. At the point of release your hand should be just below & behind your armpit.
The method I have described scoops the ball beside the body at a right angle to the normal cascade scoop. Flick your fingers into your palm as you let go of the ball to make it curl over your shoulder to the front. Push your shoulder forward so that the ball falls in front of you & not out to the side. The ball should peak just above your head & on the same side as the throwing hand.
If you are in pain it's because you didn't stretch enough.
Fitting an over the shoulder throw into a Cascade is pretty much the same as throwing behind the back. Throw the ball before a bit higher to give you a little extra time. The main difference is that an over the shoulder toss is more like an Over the top than a cascade throw because it peaks on the same side as it is thrown & is caught in the centre of the body. You may find it easier to throw over the shoulder in a Reverse Cascade.
You will probably find yourself throwing away to the side. If this happens push your shoulder forward more & twist your body to the opposite side, also swing your hand as far behind you as you can. BUT DON'T PUSH TOO HARD.
When throwing every left or every right over the shoulder stand with your body twisted to the opposite side & with your throwing shoulder hunched up. It should feel a bit like a Half Shower.
For every throw you will need to twist your shoulders from side to side, the shoulder of the throwing hand will always be moving forward & the catching shoulder will be moving back. If you are still throwing balls out to the side try flicking your ring & little fingers harder when you release the ball. Keep your head up & back, move your eyes to spot each ball as it pops up rather than flicking your head back & forth. Try to avoid throwing too high, when you do it is easy to start throwing balls so they pop up from behind your head which ruins the visual effect of the trick & you also risk getting a clout from one of the balls.
Also practise making over the shoulder throws after Claw catches, the downward snatch leads nicely into a backwards swing. Try juggling Two in one hand using over the shoulder throws (in this trick throwing to the centre isn't so important so keep your shoulder back).
For a challenge mix over the shoulder with behind the back throws, in the sequence, "Behind the back, over the shoulder, behind the back, behind the back, over the shoulder, behind the back..." It is easier to count, "1, 2, 3, 1, 2 ,3..." & remember that all odd numbered throws are back crosses & the 2 is an over the shoulder throw. This gives a pattern in which three balls pop up over the same shoulder & then over the opposite shoulder. You'll need to twist your shoulders towards the side that is doing the backcrosses & turn your head the other way to look at the balls rise over the other shoulder. Making low & tight throws so that the three balls really do follow each other is the key to making the trick look good. As I said earlier do not slip into the habit of throwing over the head.
When you get this trick solid any pattern which includes an Over the top throw or a toss to the same hand can be made over the shoulder.