I remember this issue coming up years ago, and not long after
I had the chance to carefully observe the Lees leading at a
Seattle convention. The instant of the downbeat occurred at
the bottom of the downward stroke, not the top.  I recall a
slight extra hand motion to mark the exact beat, or possibly
it was just an artifact of changing directions.

Regarding the two Youtube videos,

I carefully watched both the Homeward Bound clip:
http://www.youtube.com/watch?v=2ckxBb222Tc
and the Mount Pleasant clip:
http://www.youtube.com/watch?v=c_GSEjbMY64

In both cases the two leaders are not what you would call
synchronized, but in neither case are they at all "moving in
opposite directions most of the time." Am I watching the same
videos that everyone else is?

For example in Mount Pleasant, for every recurrence of the tenor
"Fly... fly..." both leaders have their hands down at the instant
that the "fly" melisma begins. Also for the bass fugue entrance,
both leaders have the hand down at the bottom of the stroke.

In Homeward Bound, during the second stanza after the break in
the video, the bass entrance is not clearly up or down but both
leaders clearly move to the tenors again at the bottom of the
stroke.

This video documentation coupled with my own observations tells me
that indeed in traditional _practice_ the pulse of the downbeat is
at the bottom of the stroke, not the top, regardless of what anybody
_says_ one way or the other.

In my non-SH school I was taught that the first intermediate beat
should be outward from the body, not across, to make it more clearly
visible. So the right hand motion would be drlu or dru.  When using
both hands the motions are symmetric, down-out-in-up or down-out-up.

--paulr