The spiral is inherent in flow, think of an exhale, it is all planar turbulent vortices, what are vortices? It is a balance of forces, high pressure swirls in a spiral to fill low pressure, the spiral flow creates the low pressure while high pressure surrounding the swirling low pressure flow creates the definition, the wall of the vortex. The centrifugal motion of the air drives it outward lowering the pressure of the core, while the surrounding high pressure air resists that outward force. Still air is high pressure air, flowing air is low pressure air, the still high pressure air moves to the flowing low to fill it in a spiral. The void is filled by a spiral. What is a spiral but a recursive circle? The motion of the piston is translated into circular motion, the twist is a spiral twist or engines would not need THRUST bearings. Gears are most efficent and quiet when they are beveled and spiral cut.