It's octagonal: from the Latin "octo", which means eight (8) and "n-gon" where n is the number of sides. Thus, an octagon is an 8-sided polygon, like a typical "stop" sign in the USA. Think of any 6-pt socket, but imagine it with 8 points instead of 6. Now, try to make the six-sided "tool" fit the eight-sided workpiece.
Here's a photo of the "working end" of the tool, though a bit dirty... The thing that makes this tool a necessity is that a hexagonal tool (six sides) won't work because the geometry is completely "off". Try drawing a hexagon overlayed on an octagon, or vice-versa, for it to come into focus (let us know how you do, too).
Thus, if I want to tune this blower, I have to have this tool.