A Slapback is a trick in which the juggler, instead of simply catching an incoming club, hits the handle with the flat of his hand, causing the club to spin round once, very quickly, before being caught:
This sounds painful, but looks really good. It also produces a nice rhythm when done on every throw. The idea is to make the move very quick, so that you can squeeze both a slapback and a catch into a pattern, in place of an ordinary catch.
Try it with just one club to start with. Do a normal throw from one hand to the other, and then slap the incoming club near the top of the handle, about 3/4 of the way up from the knob to the body. Did it hurt? Do this a few time and you will start to get the feel of how and where to strike the club. Next try to incorporate a slapback into a cascade. You may find that even when you manage to do it, it doesn't feel comfortable because the rhythm of the pattern has been disrupted, and you have to rush to be able to throw the 'slapped' club back in time. The time it takes you to do the trick will diminish as you improve, which goes part way towards solving the problem, you could also try throwing the club before the slap a bit higher. Also try raising your hand a little before doing the slapback, so that the impact occurs higher up, and therefore sooner, saving you time.
The main problem when trying to do continuous slapbacks with both hands (every club is slapped before being caught) is the strange rhythm involved, the only thing I can say, apart from 'practice', is to concentrate on making good throws, and not worry about the slapping itself too much.
Some jugglers prefer a less extreme version of slapbacks, with the clubs only spinning half way round before being struck:
Because this is a much more gentle move, you don't have to use your whole hand to do the slapping: fingertips, knuckles or even your forearm can be used.