pimmnz

Briefly, and in laymans terms, if the wing is on top of the fuselage it (the airplane) will roll with rudder application in the direction of the yaw. My guess is that in order to reduce this secondary effect Tom has added a bit of anhedral to try to cancel the effect, ie the anhedral tends to roll the airplane in opposition to the applied yaw. the total effect he was aiming for is that the model will yaw without any appreciable roll.
Evan.