Cracking or popping is caused by a mixture of synovial fluid located in the joint, friction and gas. The sound happens when the gas (mostly carbon dioxide) is released from the joint.
Just think about when you get a package in the mail. Often times there is bubble wrap in the box to protect your stereo, television etc. When you press into the bubble (friction), it makes a popping sound. That is pretty much like the pressure a joint gets with an adjustment. No breaking of bone, no smashing bones, just a release of gas. After the “pop” happens, the joint will not make a noise again until the gases come back into the joint. That is why once you hear a pop, another one can not be repeated.
So, now after learning this information, there is no reason to fear of the sound right? Well, if you are afraid of the sound, there are other methods of moving the joint that don’t involve a popping sound and most chiropractors will accommodate you if you are don’t like the sound.