The Cleveland Fed details:
The BLS’s Current Population Survey categorizes involuntary part-timers into those working part-time due to “slack work or business conditions” and those who “could only find a part-time job.” In other words, some workers had full-time jobs but went to part-time when their employers cut hours because business was falling off, while other workers had to start out in part-time jobs even though they were looking for full-time work, because that’s all that was available.
More specific, slack work is involuntary part-time labor as a result of a company made decision. Interestingly enough, companies haven't moved their staff to slack work hours, but have instead laid workers off or hired outside part-time workers in droves. The chart below details this showing part time hires vs. slack workers as a percent of the private work force.
I wonder if this somehow relates to full-time workers maintaining their health care benefits if hours are reduced vs. a temp hire in which the company doesn't have to pay for those same benefits?