The popular belief is that in Spey casting, the fulcrum point fixed is in the upper hand. That is not the case and the fact is that the fulcrum is not in the upper hand but at an imaginary point slightly below the upper hand.
Therefore Spey casting with a two-handed rod is simple machinery at work utilizing a combination of Class 1 and Class 3 levers.
But there is ‘more twist’ than just a Type 1 and/or Type 3 Lever. Unlike many applications of levers where the fulcrum point is static, good Spey casters apply both Class 1 and Class 3 levers dynamically by shifting the fulcrum point just slightly below the upper hand. What does it mean that throughout the casting process, the fulcrum point is not fixed in one point but moving up or/and down fluidly depending on style of the caster? In traditional style casting, at the beginning of the forward cast this fulcrum point is just slightly below the upper hand. Towards the end of the forward cast, prior to the stop - the fulcrum point could shift as far as 1/3 from the top of the overall distance between the upper and the lower hand. On an underhand cast, I believe that the fulcrum point is shifting just below the upper hand and doesn’t move as much as the traditional style.