V-shape loops are more
aerodynamic than U-shape. The way to create a V-shaped loop is to relax the hand immediately after the stop. Without relaxing the hand the rod tip continues to travel forwards creating the U-shape.
Incidentally, relaxing the hand is also important for removing bounce from the rod tip. All rods bounce to a certain extent after the stop. The better the rod the less the bounce. However by relaxing the hand after the stop, the bounce is minimalized as there is nothing to for the rod to bounce against! Orvis have a series of rods, called the Trident series, which boasts a patented dampening system beneath the handle. This is an interesting marketing concept as you already have your own dampening system outside the handle.
It is possible to vary the
position of the point in the loop. Some
casters try to position the point at the
bottom of the loop. Others put it in the
middle, yet others position the point at the
top.

It may not be the whole
story, but I haven't met anyone else who's
gotten into this stuff, although I have read
a short article on it somewhere - sorry,
can't remember where... think it was a Kiwi
fishing magazine - but the article did
nothing more than refer to the shape of the
loops and describe the point at the bottom as
a rat's snout cast, and it was a long time
ago, and I never met the fellow, but
the way I achieve the different loop
formations is by varying the angle I use to
cast the rod. Up to now I haven't used the
clock face - too restrictive - but by
casting between 12 o'clock and 10 o'clock I
get to put the point at the bottom of the
loop and by casting between 2 o'
clock and 12 I get to put the point at the
top and I can still get a horizontal
line at the top of the loop so long as I drop
my elbow in the forward stroke in order to
maintain that horizontal path of the rod tip
that I require. However this might not be the
whole story, perhaps the angle of the
backcast plays a part, and perhaps the angle
the rod tip travels as is unbends
plays a part, don't know, but when I do,
you'll be the next to find out...
One way to effectively
practice these loops is to cast with very
short lengths of line (8yrds and less).
Although opinion varies, I
prefer to position the point at the top for
most of my distance casting. This is because
the shape of this loop acts as a wedge,
keeping the line in the air for slightly
longer.
For into-the-wind casting it
is advisable to put the point as near the
bottom of the loop as possible, for two
reasons: the first is that the wind will be
less able to get under the loop, the second
is that you need that high back cast for
casting into the wind.