“This video shows a series of clips of ground squirrels responding to a spring-loaded device that uncoils toward them at the same velocity at which rattlesnakes strike,” writes Rulon Clark of San Diego State University.
All responses are shown slowed down to one-quarter regular speed. Squirrels either scrambled away from the spring (first set of clips in video), or exhibited an evasive leap, jumping into the air and contorting their bodies with the tail (second set of clips). Squirrels that had recently interacted with rattlesnakes at that site, and that exhibited tail flagging signals, responded more quickly to the device, and were more likely to exhibit an evasive leap than a scramble.
Further details are at his web site, and in the study “The fear of unseen predators: ground squirrel tail flagging in the absence of snakes signals vigilance“, Breanna J. Putman and Rulon W. Clark, Behavioral Ecology, epub 2014.
Also see Susan Milius’s essay about this, in Science News: “Why ground squirrels go ninja over nothing”