...controls the amount of "overshoot". This is described in the comments of the ActionScript sample implementation linked on Penner's site:
http://robertpenner.com/easing/penner_easing_as1.txt
/////////// BACK EASING: overshooting cubic easing: (s+1)*t^3 - s*t^2 //////////////
// back easing in - backtracking slightly, then reversing direction and moving to target
// t: current time, b: beginning value, c: change in value, d: duration, s: overshoot amount (optional)
// t and d can be in frames or seconds/milliseconds
// s controls the amount of overshoot: higher s means greater overshoot
// s has a default value of 1.70158, which produces an overshoot of 10 percent
// s==0 produces cubic easing with no overshoot
...controls the amount of "overshoot". This is described in the comments of the ActionScript sample implementation linked on Penner's site:
http://robertpenner.com/easing/penner_easing_as1.txt