Hmm. I don't know it as a musical symbol per se. I've seen it on architectural drawings to indicate that the nearby measurement refers to the line segment that the arrowheads define, and in written works to indicate that the sentences on either end of the arrowhead line should be swapped. Anyone else?
It's used in Logic as an icon on audio files that contain tags generated from analysis and transient detection that allow for real-time stretching. You'll see it on Apple Loops and on audio files in the Arrange Window where Follow Tempo is being used.
Hey Eli -
That answers that question. Thanks man. I've noticed that when that icon exists on an audio file that I've just stretched in flex time, it then play like it has a modulator effect on it. Like a leslie, which is bad. Do you know what causes that?
Seriously though, flex isn't a magic bit of miracle. It's a tool that has it's limits. Experiment with the different algorithms, edit the detection process and resultant transient markers. And even still, with all those tweaked optimally,there are limits to how far you can stretch. I'm afraid there's no one single magic answer for you.