picked up a poi last night, waggled my hand in a circle for a bit and this is what i figure:
horizontal plane isolations require a constant force by your hand to keep them going.
once you have enough speed, they will stay pretty flat.
vertical isolations are trickier for a very good reason - gravity is fukkinwit'choo.
with the example of a forwards (vertical) isolation, more force is required when your hand is on the top front part of the circle.
this is because the poi will be on its way up (on the other side of the circle) and working against gravity.
hence vertical isolations are a bit trickier.
but it does give you the choice of giving your isolations a 'clunky' quality.
a friend mumbled something about 'isolations with a beat' to me a while back and this optional 'clunk' (the bit where you start applying force to the motion again) is what i would say he was on about.

back to the balls...