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.'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.