the 'jerk' is happening because you have one more frame than is necessary.
when you've rotated 15 degrees, and then set back to 0, basically your doubling
two frames, the one at 15 and at 0. to remedy, if you are rotating over 50
frames, the increment of rotation is 0.3 so on the 50th keyframe reduce the
rotation amount by 0.3, from 15 to 14.7
there you go
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