This is a nice one to pull off every now & then, juggle, throw two balls up then pass the third from hand to hand behind your back & carry on juggling.
Passing a ball behind you is pathetically simple but for the sake of completion, take one ball & hold it in one hand. Stand with your feet shoulder width apart & with your arms hung loosely by your sides. Now in one movement, lean your upper body back, rock your hips forward & swing your two hands together behind you. Pass the ball over & rock back to standing upright. Practise this both ways a few times just to familiarise yourself with the motion. Notice that throughout the movement your head stays in almost exactly the same place. This isn't really important when passing underneath really High Throws but can help when you want to make a pass under throws that haven't passed up above your head. Keeping your head still will make it easy to know where your body is in relation to the pattern, spatial awareness is a lot easier if you're not moving all over the place.
Now take all three balls & juggle a cascade, then throw in one round of 441. If you can't already juggle 441 it doesn't really matter, all you need to know is the basic idea of the trick (but learn it anyway). Juggle your cascade then throw one ball straight up in the air to above head height, then throw the next ball straight up to the same height on the other side of the pattern with the other hand. Then while the balls are in the air pass the third ball straight across from the hand that threw the first vertical throw into the hand that threw the second vertical throw. Catch the first vertical throw with the same hand that you threw it with, throw the ball you passed from hand to hand as a cascade throw, catch the second vertical ball & resume the cascade.
Do the same again but this time pass the ball behind you, just like you practised, instead of in front of your body. Throw the two vertical throws up high & one after the other in quick succession, to give yourself plenty of time to comfortably make the pass.
For greater enjoyment gradually reduce the height of the throws, you can also put in a behind the back pass into any siteswap pattern that contains a 1, 441, 531 & 45141 are good ones.