Unchecking "Y" in the first "Copy Rotation Constraint" for "DEF_arm.006" at 22:14 with the "Influence" of the "Constraint" which copies the rotation of "MCHarmSpline.006" set to "0" causes strange movements of the "Flight Feather Deformation Bone Chains" "DEF_fthrFlightH" to "DEF_fthrFlightH":
Compared to the situation in this lesson's video:
