Answer
	You hear knappers talking so much about "the cone" of percussion, because it is the physics principal that governs the flake removal properties in flintknapping.  If you've ever seen a window that was struck by a B-B or been the unfortunate receipient of a rock into your car windshield, you've witnessed what the whole bulb of percussion looks like, or in technical terms, the Hertizion Cone.  If you could study the cross section of the glass where the B-B or rock impacted the window, you would observe that the impact area would be the narrow end of the cone while the part nearest to the inside of the glass would be the widest part of the cone.  This can be explained simply by saying that the percussor (the B-B or rock) struck the the glass and the energy spread out from the impact area inward until the energy of the percussor strike was expended.  Had the B-B or rock had more energy, it would have continued to travel through the glass thus breaking it, but under the right conditions, the percussor had just enough energy to initiate energy into the glass, but not enough to break it.  The resulting bubble or bulb is like a permanent recording of the energy transfer between the rock and the glass.      Wow, that was probably a lot to digest, and about now your probably thinking "Alright, but how does this apply to knapping?".  The cone pertains to flinknapping because every time you strike a piece of flint or chert the resulting flake represents approximately half of the Hertizion Cone.  The cone shape is always consistant, and the angle you hold the piece you are knapping will determine how much of the partial cone that will be carried through the piece of flint or chert.  The steeper the angle that you hold your piece of flint or chert, the longer the resulting flakes will be that detach.  If you percussion knap on your thigh, the further down the outside of your thigh that you hold the piece, the longer the resulting flake will be.  Again, more of the length of the cone is being applied to the piece of flint or chert.  If you place the piece straight on top of your thigh, a short steep flake will result due to catching less of the cone's angle length into the piece.  This is why it is so important that your swing remain consistantly at the same arc of travel so that your billet strikes the piece at the same striaght 90 degree angle each strike.  Remember........it's your piece that changes angle, not your billet strike angle!!  You really want there to be only two  variables: the angle you manipulate the piece of flint or chert, and the physical qualities of the stone.  Adding anymore variables just makes flintknapping all the more frustrating.  The cone will become more understandable the more you knap, and it will become your friend.
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