Natural movement tends to be sinusoidal. Consider a figure swinging its leg forwards then back. It will start off slow, speed up in the middle of the swing to a constant velocity then slow down at the end. Poser enables this type of movement by using the Spline Section. If you used Linier then the movement will tend to look mechanical or robotic. The difficulty comes when you want to swing the leg back again. Poser interpolates the curve based on all the available key frames. In this instance you'll have one at the start, middle and end of the movement. If you alter the value of the backward swing key frame you will find that you are affecting the start of the swing which you may not want. The solution is to use a spline break on the forward swing key frame. This effectively isolates the movement into two sections. The swing back will not affect the swing forward.