S Curve for V4 & V6 Siphon

S-trap is a curved hydrodynamics device designed to facilitate minimizing siphon as long as possible so that maximum siphon is achieved through a sudden jump in the water velocity.  It operates fundamentally different from the 90-degree elbow. The scientific method here is Stick and Slip phenomenon. Stick-slip can be described as surfaces alternating between sticking to each other and sliding over each other, with a corresponding change in the force of friction. If an applied force is large enough to overcome the static friction, then the reduction of the friction to the kinetic friction can cause a sudden jump in the velocity of the movement.  You can see the water column inside a bell rise in stick and slip motion fashion.