Millennials are playing with fire — or, rather, 'FIRE,' an acronym for a new trend in personal finance which stands for "Financial Independence, Retire Early."

A small subset of millennials in the 'FIRE' movement are living frugally and stashing away large percentages of their incomes, sometimes as much as 70 percent, so they can retire in their 30s and 40s.

Historian Nancy Koehn joined Boston Public Radio to put the trend in perspective. She said that 'FIRE' stands out from past counterculture financial movements because it depends on its adherents playing the market wisely and not dropping out of it entirely.

"This is an important distinction between other movements that are about really downsizing your life and cutting back on consumption for other reasons," Koehn said. "These folks are about investing really intelligently in the stock market."

"That's just very different from, say, some of the hippies or movements in the 60s that wanted to go back to the land and ... said, 'We're done with the consumerism and the 'rat race,'" she continued.

Koehn said this movement is less about shunning consumerism and more about control — perhaps a reflection of a growing percentage of Americans who say theyfeel completely disconnected from their jobs.

"A big hunk of this is about getting back your time and energy," Koehn explained.