Slashing and spreading is used to add fullness to a piece of cloth by, well, "slashing" the pattern tissue into several panels, and then "spreading" those panels out. It's a neat way to adjust a plain shirt pattern, as I'm doing this week:
Once I had the strips cut, I placed them on a large sheet of newspaper. For the upper section - the yoke - I wanted there to be loosely gathered "tucks" running from the shoulder line to the bottom seam, so I simply added about 0.5" of space between each panel. But I wanted the body piece to be fuller at the bottom, so I rotated each strip, allowing the pattern to spread out in a fan shape.
After taping down the strips lightly I traced a new pattern, and that was that!
Then, of course, came the actual cutting of the cloth. I used a very light silk for the yoke, and a loosewoven, slightly heavier cloth for the body. Any lightweight fabric that drapes nicely would work well for this technique: silk, light linen, cotton, and single-knits are ones that come to mind.
Time to sew!