[PyMOL] animate angle bending

Dear pymol users,

I'm struggling to figure out how to create an animation of an angle 
bending vibration
with pymol, like this one 
http://www.ch.embnet.org/MD_tutorial/Images/Part2/a.gif
My question is how do I correctly set the origin and axis so that I 
could use the rotate command
to rotate one bond with respect to the other.

Thanks a lot in advance and best regards,
Sven