

(The feather in the hat is a jiggle bone)

The moving hat? Tahts what it does.
Facepunch link with feather hat: http://facepunch.com/showthread.php?t=1275953
Explanation of jiggle bones: https://developer.valvesoftware.com/wiki/$jigglebone
I think it'd be pretty cool to see some in hl2:dm if possible.
Just need to find models with them.
cl_jiggle_bone_framerate_cutoff is default at 45 so if your under 45 fps all jiggle bones become static.
Setting this value to 0 will make them always be on.