This "quick tutorial" introduces the general idea of a call and explains the most common cases. It does not explain or illustrate all cases. For additional details and/or examples, refer to the links in the "MORE INFO" box.
Chain Reaction starts in a formation like this. Notice that one dancer in the center line is directly facing a dancer on the outside.
Those two dancers Pass Thru. The dancer from the center line meets the other outside dancer, and those dancers do a Hinge. Meanwhile, the dancers at the end of the center line walk forward 1/4 of the way around the outside of the set, to meet one of the dancers that did a Hinge.
As a result of these actions, there will be a center star of four dancers and two pairs of outside dancers. The dancers in the star move forward 1/4 (one position) in the star, while the outside pairs Trade.
At this point, one of the outside dancers will be meeting one of the dancers from the star. Those dancers take hands and Cast Off 3/4, to become the centers of new parallel lines. The other four dancers "move up" to become ends of the lines, in the same way as four dancers move up to become ends of lines in a Coordinate.
Of course all of these actions take place immediately following each other. The dancers don't stop at each position.
This isn't actually as complicated as it may sound, because each dancer isn't doing all of these things. You may find it helpful to review the action in this example from the perspective of the dancers in each of the four starting positions. You can use the animation to follow each starting position in turn.
Note that when Chain Reaction is done from a right-handed 1/4 tag, the hinge will be by the left, the star will be right-handed and the trades will be right-handed, the cast off 3/4 at the end will be left-handed, and the resulting waves will be right-handed.
If Chain Reaction is done from a left-handed 1/4 tag, the initial pass thru is still done passing right shoulders, but everything else will be reversed. The hinge will be right-handed, the star and trades will be left-handed, the cast off 3/4 will be right-handed, and the resulting waves will be left-handed. It is especially important in this case for the dancer on the outside to wait and actually do the hinge with the person coming from the center -- if they aren't both very familiar with this case, and don't actually do the hinge with each other, there is a good chance that one or both of them will end up in the wrong position.
Another common problem that beginners have with Chain Reaction is the distance to move in the star. Each person only moves 1/4 (90 degrees). This is a very short distance. Until you become accustomed to the path, you may want to take this part very slowly and deliberately, to make sure you don't overshoot.