A 'backcross' is juggling terminology for a behind the back throw, with the club coming up over the opposite shoulder. It sounds simple but the throws must be quite accurate, and at the point of release the club is completely hidden from view which means you have to wait for the club to appear before judging whether it is on track.
The best way to throw is to lift the club with your arm, rather than flick it with your wrist. You may find that shrugging your shoulder helps your arm with the lifting movement. Again it is very important to hold the clubs near to the ends of the handles for this trick.
Backcrosses are commonly done with 1,2 or 3 spins.
For single spins you will probably have to move your head from side to side to see the clubs coming over your shoulders, it is also necessary to throw forward quite a bit, rather than just straight up, so that the clubs come into your field of view more easily. Carry the clubs over to the centre of your back before making a throw. If you want to try just one or two backcrosses, rather than doing them continuously, then singles are probably the easiest ones to try.
With double spins, backcrosses are a lot slower than with singles, and for this reason I would say they are the easiest choice if you want to try the trick continuously. You only have to throw forwards a tiny bit on doubles, because they are in the air for so long that even the slightest forward motion is enough to bring them over your shoulder by the time they land. Look straight up, you don't have to move your head as you do with singles. The best place to release the club is about half way out from the centre of your back on the side that you are throwing from.
On triple spins this trick is very slow indeed, but because you are throwing so high you must be reasonably accurate. Don't try to throw forward very much or the clubs will land too far in front of you. Your hand doesn't have to move very far across your back to make the throws on triples, the clubs come up from behind your head or over the near shoulder, as soon as you have got your hand behind your back lift the club straight up to throw. Practice juggling 3 clubs on triples for a while to get used to the rhythm, the timing is the same when you backcross them.
I think single backcrosses look best when viewed from the side because of the forward motion of the clubs. When doing doubles and triples, for full effect face the audience.
Because double and triple backcrosses are so slow, it is possible to incorporate an extra swinging movement into the trick making a very nice combination. Each time you catch a club, swing it in an outside circle and then release it for the next throw. The swing should be done just with the wrist, keep your hand down low and your arm straight. Try to start the swing with one hand at the same time you start the throw with the other. Alternatively throw a double backcross slightly higher than normal, catch it with your palm facing outwards and flourish it before you throw it back - this looks wonderful when done continuously. The main difficulty is that once the flourish is complete, your hand may be too far up or down the handle to do an accurate throw. I find the best way to resolve this is to grab the club near the centre, so that the flourish doesn't need much power to get it round, and then slide my hand right down to the end of the handle for the throw.
It is also possible to flash 3 clubs on backcrosses, using doubles or triples. Start by juggling singles, either behind the back or just in a cascade, and then throw the clubs one after the other as quickly as you can up behind your back. A common mistake with flashes is not to put enough spin on the last club, so watch this one in particular. A nice challenge for the more advanced jugglers is to do the flash on triples and as soon as you catch the clubs, flash them back up again. This is good practice for 5 club backcrosses (or so I'm told!).