slow in and out

This refers to the spacing of the inbetween frames at maximum positions. It is the second and third order continuity of motion of the object. Rather than having a uniform velocity for an object, it is more appealing, and sometimes more realistic, to have the velocity vary at the extremes. For example, a bouncing ball moves faster as it approaches/leaves the ground and slower as it approaches/leaves its maximum position.

The name comes from having the object or character "slow out" of one pose and "slow in" to the next pose. In early animation, the action was limited to mainly fast and slow moves, the spacing from one drawing to the next fairly even. But when the poses of pose-to-pose animation became more expressive, animators wanted the audience to see them. They found that by grouping the inbetweens closer to each extreme, with only one fleeting drawing halfway between, they could achieve a very spirited result, with the character zipping from one attitude to another. "Slowing out" of one pose, then "slowing in" to the next pose simply refers to the timing of the inbetweens.

In most 3D keyframe computer animation systems, the inbetweening is done automatically using spline interpolation. Slow in and slow out is achieved by adjusting the tension, direction or bias, and continuity of the splines. This works well to give the affect of slow in and out, but a graphical representation of the spline is required to see the effect of tension, direction, and continuity have on its shape. With this type of spline interpolation, a common problem is the spline overshooting at extremes when there is a large change in value between them, especially over a small number of frames. This also happens when the direction control of an extreme is adjusted. The danger is that, depending on the variable the spline controls (translate, rotate, or scale), the value will shoot in the wrong direction just before (or just after) the large change in value. Sometimes this effect works out well when it occurs just before a large movement it may appear to be an anticipation. However, more often than not, it gives an undesirable effect.