this same thing was observed with sharptail and harriers. They roll in rocking back and forth all non-nonchalant then make a very aggressive sprint to the grouse and go for a bronco ride. I have seen them do it on pheasants...
There was a falconer in the mid '80s here in Washington who goofed off with a harrier one season and caught a few pheasant with it.
Geoff Hirschi - "It is better to have lightning in the fist than thunder in the mouth" Custom made Tail Saver Perches - http://www.myrthwood.com/TieEmHigh/