Adding to what Larry has posted, the Belgian cast and reverse Belgian cast are also called constant tension oval casts. The standard Belgian cast is a low sidearm backcast with an immediate transition to an overhead or higher sidearm, forward cast. The rod tip goes in an OVAL path without a stop, hence the name “constant tension” cast and “oval” cast in addition to the name “Belgian” cast. The reverse Belgian is a high sidearm or vertical backcast connected to a lower sidearm cast.
The reverse Belgian is used when casting INTO the wind. When casting INTO the wind, the higher back cast uses the facing wind to add energy to the back cast and the low forward cast takes advantage of the lower facing wind nearer the ground or water surface. So the high back cast is WITH the wind and the lower forward cast is against the wind.
The Belgian cast is used when there is no wind or when casting with the wind. The lower back cast is made against the wind and the forward cast is with the wind which helps lengthen the cast and extend the leader.
Since the back cast and forward cast are in different casting planes, there tailing loops cannot tangle the line or leader. Even if the paths of the fly line/leader of the back cast and forward cast cross each other horizontally, they are separated vertically and CANNOT get tangled. For a the fly leg of the line to hook the rod leg of the line, they MUST be in the same casting plane. So the Belgian cast is a cure for a tailing loop.
Since there is NO STOP in the cast and the paths of the back cast and forward casts are separated, the oval cast is used when casting heavy flies or rigs that would jerk the rod on the rod stop between the back cast and forward cast. It is the cast to use when casting heavy streamers, rigs with split shot, dry dropper with heavy nymphs, etc.. It helps prevent the rigs form hitting the rod tip or shaft and breaking the rod. It is also used when casting heavy lines like sinking fly lines.
Casting Heavy Flies | MidCurrent
The Belgian Cast | MidCurrent
How to cast weighted fly line, heavy flies, and split-shot
However, there is a disadvantage to the Belgian cast. Because it is an OVAL casting motion, it introduces another twist into the line for every oval casting motion
. If you adopt the Belgian cast as your standard cast for everything, you need to allow the fly line to UNTWIST, by letting it drag in the current, With a light fly, the line will untwist, BUT with a heavy fly, it will not.
So be attentive when you use the Belgian cast to monitor your line for twist. Reeling up a twisted fly line and storing it that way is not good for the line. After you take off the fly, let it untwist before putting the line back on the reel.
"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."
The Belgian Cast
Here are videos of the Belgian Cast