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.
But the most common uses of Spread differ enough that grouping them into three different cases is helpful in explaining them.
From a squared set...When the caller asks the heads or the sides to do a call that has them turn 1/4, e.g., Slide Thru, they end up in the center of the formation, facing or back-to-back with the other dancers, the ones who were not doing the call. Adding Spread to this kind of call means that the dancers who were doing the call should move apart sideways, allowing the others to step forward to become the centers.
For example, after a Heads Slide Thru, the head dancers would be in the center, facing side walls. To do Heads Slide Thru and Spread, after they do their Slide Thru the heads move apart sideways, while the side dancers step forward.
Of course the same could apply to a call done by the sides, e.g., Sides Touch 1/4 and Spread. The heads will be stepping forward into the center, as the sides move apart sideways after doing their Touch 1/4.
Calls that end with one couple in front of another...Adding Spread to a call that would normally end with one couple standing in front of another means that the dancers in front should move apart sideways, becoming the ends of a line, and let the ones behind step forward to become the centers of that line. For example, Wheel and Deal and Spread.
Calls that end in a wave...When Spread is added to a call that would normally result in a wave, it means that the dancers who would have finished as the centers of that wave instead move to the end positions, and the dancers who would have finished in the end positions move to the center positions. For example, Swing Thru and Spread.
Note the the dancers finish as if they had done the first call and then moved past each other "nose to nose", but they don't actually do it that way. When possible, they blend the switching of places together with the last action of the first call. They anticipate their modified destination and move to that spot as part of finishing the first call.
Depending on the call, they may be able to do this without any separate sideways motion at all. For example, to do a Spin the Top and Spread, the end and center dancers can switch places as the same time as they are walking forward around the middle of the wave.