Nate wroteSomething is wrong with your rotation. The arm shouldn't snap around like that. Note sure exactly what it could be...
Ok, I've finally dug into this a bit more.
I think that less than 30 frames of my animation are being drawn, but I'm not sure why.
I've got an animation with 120 frames. Only part of the animation ever draws with the Python, so this is what made me start thinking that this bug may be related to the # of frames.
Sure enough, I can cause a snapping arm movement with Spineboy by looping @ < 30 FPS.
Here's what my sword example should look like:
http://youtu.be/06wn68e0D7M
Here's what it actually looks like:
http://youtu.be/mKPTzDb4P8g
Seems like it might be timeline related, or maybe it's an issue with the values I'm passing to apply? What do you think?
[Edit] In case it's not obvious, the Spine recording wasn't playing from frame 0, but I recorded enough of the video playing for you to see the animation start over @ 0... sorry about that.