4 axis machining

As I mentioned in another thread on the building of a small 4th axis
I want to try my hand at 4 axis milling. To start with I am going to
do some simple shapes in foam using a stepper motor direct drive as
the rotarty table before moving on to something with higher
resolution etc.

Over the weekend I thought I would have a play with mastercam and see
how much there was to this 4 axis milling lark. I drew a sphere in
Rhino and imported it into Mastercam. To my suprise I soon had a
tool path that would cut the sphere in a revolving block although
master cam shows the tool revolving in the simulation.

I then posted this to turboCNC and let it rip to see what happened.
I expected TCNC to complain that I had not set up a 4th axis but it
didn't so I let it go as it was. All three axes moved but not in the
way I expected. I expected a constant revolution from two of them
one the "rotary" the other the X axis and an up/down movement from
the z.

This didn't seem to happen and all three axis moved in one direct
only.

I'm very confused by it and want to get to the bottom of the probable
cause. Is it to do with the way I set up the tool path? Could it be
the post processor? Why doesn't the gcode demand a 4th axis to be
present?

I'm really just wondering if I am missing fundemental knowledge about
4 axis machining.

Thanks

Graham