Bela Fleck and the Flecktones is an American band that combines jazz and bluegrass music. The band's name is a play on 1960s rock band Dick Dale and the Del-Tones. The Flecktones formed in 1988 when Bela Fleck was invited to perform on the PBS TV series The Lonesome Pine Specials. The original members were Fleck on banjo, Victor Wooten on bass guitar, his brother Roy Wooten on Drumitar, and Howard Levy on harmonica and keyboards.

In April 1997, the Flecktones became a quartet again, as they added saxophone player Jeff Coffin to the mix. Coffin would remain in the band from 1997 until 2008. In June 1997, the Flecktones opened for The Dave Matthews Band. In July the Flecktones toured Europe and sat in on the second set of a Phish show in Lyon, France. This marked the second and final time that the Flecktones played with Phish.  In June 1998, the Flecktones released their sixth album, and fifth studio album, Left of Cool. Left of Cool represents a switch from previous Flecktones albums, as the band, according to critic Terri Horak "jettisoned their self-imposed rule to only record what could be duplicated on live instruments." In July 2000, the Flecktones released their eighth album, Outbound. Outbound is another studio album, and again the Flecktones' philosophy with this album was to do something different from everything they had done before. What makes Outbound unique is the way in which the Flecktones recorded the album. The quartet recorded each track on the album, then invited guest musicians to overdub vocals or instrumentation. Outbound guests include: Jon Anderson from Yes, Shawn Colvin and John Medeski, of Medeski, Martin and Wood, to name a few. The album earned the Flecktones the Grammy for Best Contemporary Jazz Album that year.  Live at the Quick, which was also released as a DVD, is the band's ninth album, and second live album. For fans of live Flecktones, this album, like Live Art, successfully captured the sound and feel of the Flecktones in concert. Little Worlds, the band's tenth album was released on August 12, 2003. Like previous Flecktones' albums, Little Worlds features a slew of musical guests, from Sam Bush on mandolin, Derek Trucks on guitar and even former New York Yankee and guitarist Bernie Williams works his way into a track. Little Worlds was released as a 3-CD album, but for the less committed listener, the band also released Ten From Little Worlds, which includes ten songs off of the original 3-CD Little Worlds.  In February 2006, the band released their eleventh album, The Hidden Land. As with every Flecktones album, they needed to change something from their last album. For The Hidden Land, the Flecktones didn't want any guest musicians. "'The truth is, the last few records are not what we are,' Fleck said. 'Obviously, we loved playing with those musicians, but if you keep on doing it, you become a gathering point rather than a group with its own identity.'" For the Flecktones to keep moving forward, they felt their music had to get back to the roots of the quartet. After spending much of the year apart, the Flecktones came together in 2008 to release a holiday album, Jingle All the Way.

Using a quote from the above article, answer the following question: What was unique about Left of Cool?
the band, according to critic Terri Horak "jettisoned their self-imposed rule to only record what could be duplicated on live instruments.