Answer: Electricity. According to an American Chemical Society press release, the scientists "connected a powerful electrical amplifier to a wand-like probe and used the device to shoot beams of electricity at an open flame more than a foot high. Almost instantly, the flame was snuffed out. Much to their fascination, it worked time and again." One of the scientists, Ludovico Cademartiri, said the phenomenon is not totally understood but believes the electric current destabilizes particles of burning soot, causing the flames to dissipate.
For more, read the ACS press release at: http://portal.acs.org/portal/PublicWebSite/pressroom/newsreleases/CNBP_026931