A club is dropped down behind the juggler's back, caught blind with the other hand, and thrown back into the pattern. This is really a fast version of the previous trick.
To make that catch a bit easier, try to drop the club so that it touches your back lightly as it falls, giving you more of an idea where it is. You can also use your back to help your hand with the actual catching - try trapping the club between your hand and back to slow it down before actually grabbing it.
An extension to this trick is also possible: instead of bringing the club round to the front after catching it, try to stuff the body between your legs. This can be done fairly quickly because your hand doesn't have to move very far. From this position either grab the head of the club, which will be poking out from between your legs at the front, and continue juggling, or allow the club to fall down onto your foot, to be kicked back into the pattern.
The easiest way to incorporate a backdrop into a juggle is simply to substitute it for a normal cascade throw. However during a backdrop the club only takes a split second to travel from one hand to the other - a lot less time than a cascade throw would. So in order for the receiving hand to be free at the right time, you should delay the backdrop for as long as you can. Hold the club in position behind your back, and wait until the last possible moment before dropping it, and bringing your hand back to make the next catch. To buy a little more time for all this to go on, make the throw directly before the backdrop a little higher than normal.
This might all sound like rather a lot to think about, and if you just want to practice the trick and not worry about the details, then feel free to do so - as is the case with any trick, you will eventually find yourself making the necessary adjustments anyway, without thinking, but it can often help to know just what you want to do before you start.