lamb's tails

[Barnhart]

What constitutes lamb's tail shakes.  When folks say two shakes of a
lamb's tail, they mean very quickly.  I have seen lambs shake their tails.
 But I don't think I've ever seen just two shakes.  And, does a shift to
the left and a shift to the right account for one shake or two?  I know
that this is a frivolous question, however, it's been nagging me ever
since my 13-year-old son asked me.

Regards,
David

barnhart at highlands.com