Spline mode approximates a smooth animation curve, aligning the handles accordingly. Play back the ball's animation again to get a sense of how this differs from linear mode. While splined tangents work well most of the time, they can also have some potentially undesired side effects. The most common is that Maya will overshoot attribute values in order to make a smooth curve. In this example, the lowest key values represent the bouncing ball hitting the ground. The spline causes the ball to go through the ground - an effect most noticeable after the second keyframe.