Cubic uses the equation
y = -2kx^3 + 3kx^2 +
(1-k)x. It has fairly limited easing.
x/|x| sigmoid uses the equation
y = x / (1 + k*(abs(x)
- 1)) (and its inverse
(1+k)*x / (1 + k*abs(x)) for
k). It goes all the way to a full step function
in both directions.