Poser uses spline interpolation by default.  The movement at a keyframe will be exactly what you specify BUT the position at other frames depends on the curve calculated to join the keyframes.

This leads to things overshooting or going in odd direction when you just wanted a stop. Thats where break spline comes in , set breakspline whenever something stops or reverses direction.  This is not quite true for all cases. You can use extra keyframes to control the spline.

IK is not an animated parameter. Is either on or off for the full animation.