Riffing was mainly done while witnessing. It was an exaggerated put-on (putting somebody on – sort of mocking them) if they weren’t serious while you were witnessing to them. Also, it was done at times to get a crowd gathered so you could witness to the more open interested by-standers who were nearby. It involved silly, loud antics.The Stewart flop  might qualify. It was mostly done by two or more brethren, usually brothers. It was definitely a Stewart-taught method. There may have been other ways this was done but that is how I remember it.
(Contributed by Steve Saxon)
From Mike Montoya’s History of COBU: Members would “riff” or create a dramatic scene in order to spark patrons’ interest in the gospel.
I also remember in the early FF days going to diners and holding a kind of predetermined dialogue with another brother or sister in a booth about salvation (like “did you ever hear of being born again?”), loud enough for the other people in the diner to hear. Then other brother or sister would ad-lib “No, what’s it about?” Then the other would open their Bible say something like, “Well in John 3, a religious leader named Nicodemis comes to Jesus at night…” So you continue to play off one another. You get the point. So you had a practically captive audience who got to hear the gospel.
I remembering witnessing at a college campus during the FF days and claiming that Jesus Christ was a real and historical person and in the course of the deteriorating conversation, the student eventually brought up the popular Eastern philosophical belief that we can’t even be sure if our existence and all existence was nothing more than a illusion (maya) – which by the was was later reintroduced to our culture by the “Matrix” movies. I replied, “If I kicked you in the shins, would your painful response be an illusion? Let’s see!”
(Contributed by Herman W.)
 The Stewart flop: (Yet to be defined – I’m looking for a good explanation of this. Essentially, Traill would collapse on his back in public, and after a crowd gathered around him, he’d jump up and preach from the bible.)