I believe Brian has it right. It is a constant tension "Belgian" cast but the oval is tight on the backcast.
Here is a video that shows the cast:
Here is a description:
http://www.flycastingschool.com/The_Belgian_Cast.pdf
One disadvantage of this cast is that since the rod goes in an oval, it introduces a twist into the line. Also studies have shown that the oval cast is harder on the elbow and wrist than the overhead style of casting.
http://www.working-well.org/articles/pdf/Fishing.pdf
"A study looking at pain ratings in relationship to casting style found no significant difference between casting style and shoulder pain. However,
Elbow pain was significantly less in casters who used multiple casting styles.
Elbow pain was significantly less for the overhead style compared with the elliptical style.
Wrist pain was significantly less for those who used the overhead style instead of either the elliptical or sidearm styles.
Wrist pain was significantly less for those who used multiple styles."
The take home lesson is that the overhead casting motion is better ergonomically than elliptical or sidearm casting for the elbow and wrist. The shoulder being a ball and socket joint is designed for rotation so none of the casting styles was any worse than the other for shoulder pain. I think the reason for elbow pain is that the elbow is a hinge joint and is designed to flex and extend in a single direction that is best suited to an overhead motion that places no rotational stress on the elbow.
Similarly, the wrist and the muscles of the forearm are strongest in varus and valgus flexion. That is why we hold a hammer and pound with a karate chopping motion rather than with the palm facing forward in a patting motion. Any motion of the wrist that requires a rotation using the patting motion will stress the wrist.
However, pain can still occur if you are locked into a single casting style, so mix it up.