The TinyG post files simply define the value of this variable and never uses it, so I assume that it is used by the overall g-code processor to change its behavior.
In practice, I have compared gcode files; with this change, it generates a very long list of straight lines. Without the change, it issues a simple G2 or G3 command.
I did not come up with the solution (obviously!), read it elsewhere. The issue was described as the Arduino does not have sufficient processing power to compute the arcs on time, thus mangling the path occasionally. From my experience, the “defect” where very reproducible (e.g. the 6th and 7th pockets in a long list). Never had any issues after.
Note that in Fusion 360, they updates the post from time to time. You will have to reapply this patch occasionally to their latest TinyG version.
