From PROBOTIX :: wiki
G64 is path optimization. It will perform look-ahead and calculates the angle of the next segment, only slowing down as little as possible to round the corner and not violate the velocity and acceleration settings.
It is turned on by default (look for it under active g-codes under the MDI tab), but it does not have a P parameter (tolerance) by default. You want path optimization, but you need to give it a tolerance - otherwise LinuxCNC will take its liberties with the tolerance and round over your corners when cutting at high speed.
Type G64 P0.010 into MDI - this will tell LinuxCNC to yes, turn on path optimization, but do not deviate more than 0.010" from where I told you to go. The tolerance you use will depend upon the types of parts you are making. I will set 0.010 or more for decorative type things, especially while 3D carving. For more precise parts, I will use 0.001.
One more thing... You can put that G64 line in the preamble of your post processor so that it inserts it into every g-code file. It is modal, so it stays the same unless explicitly changed with another G64 Pxxx, or canceled with a G61.