Put quite simply this cast is
a side cast in the back cast, followed by
an overhead cast in the forward. It is very
useful for (a) acquiring distance very
quickly (b) keeping flies apart on a
multi-fly leader and (c) casting with the
The disadvantage of this cast
is that it throws a half twist in the line
every cast. Half twists add up! So it's best
to use this cast sparingly, otherwise you
will have to get into the habit of removing
the reel from the rod, every 30 casts, or so,
in order to spin the twists out.
partner of mine, who uses this cast for most
of his fishing, has the infuriating habit of
casting his line out, unspinning the twists,
refitting the reel (all very casually you
understand) and bringing in the slack only to
discover he has hooked a fish, which he
promptly lands with deft skill.
One excellent reason for performing this cast is that (with slightly too much power), once the loop has straightened out on the backcast, it will kick around to the side. With an overhead backcast the loop will flip over downwards.
A side cast is excellent for casting narrow loops which makes this cast perfect for casting with a following wind. A tight low backcast followed by an open forward loop. Remember to haul as you stop on the backcast, and through the stroke on the forward cast.
Since the backcast can be presented very low to the water, this is an excellent cast for making a high forward cast.