Hey guys,
I was wondering what actually creates the pitch and roll coupling with rudder application on most aerobatic planes?
Thx

Ostensibly, the purpose of a rudder is to yaw an aircraft. To do this, the control surface on the rudder moves to create lift in the opposite direction of the intended yaw.

Look at the point at which the rudder develops this force. The yawing force is typically centered above the centerline of the aircraft. Well, if it is above the centerline of the aircraft, it's not too different compared to how an aileron works on the main wing. Basically, it is force at a distance... torque... which tends to roll the aircraft in the direction of the lift force.

As for pitch coupling.... induced drag increases when lift increases. It's a similar phenomenon as the above. When you apply rudder, you get increased lift on the rudder with it's associated increased drag. This drag force is above the centerline of the aircraft and points to the rear. Think of it like a shift lever. It pitches the aircraft up... in this case, it doesn't matter whether the rudder deflection is left or right.

This is a simplistic explanation as there are more interference and coupling effects that can take place.

I had heard that it involved a pressure differential across the horizontal stab... is this part of it?

Most aerobatic planes pitch down with rudder, and most roll the same direction as the rudder is applied.

That's getting into the more complex (!) explanation and has to do with interference of airflow over part of the horizontal stab (essentially making it less effective). So yes, that very well could be the case with particular aircraft. It really has to be examined on an case by case basis.