Slow In and Out

Slow In and Out has to do with the acceleration of the object being animated. An object can't go from standstill to a speed without accelerating. As a car stops it does so slowly and slowly speeds up. Although the use of "slow" in this case may not be the best word to use, the idea is simple, for example a bouncing ball. When we bounce a ball there are only two times that the ball is not moving for just a split second: at the top of the bounce and the bottom of the bounce. As the ball approaches the top it slows down till it stops (Slow In). Once the ball has stopped for that micro second it is not moving. Then the ball starts to accelerate to the ground again because of gravity, but it is done slowly (Slow Out).

As it turns out this type of motion is repeated for all moving under acceleration objects. Sometimes the motion is so fast that it can be ignored like a gunshot but with some things is would look unnatural with out the slow in slow out.

Here is a good example of this process.

The ball on the left moves at a constant speed with no eases and no squash/stretch.
The ball in the center does slow in and out with squash/stretch.
The ball on the right moves at a constant speed, no eases, but with squash/stretch.

Now with the timing sped up, the sacing favouring the top even more so the bounce impact is snappier, you have this verion with slow in+out at the top pose, and slight squash+stretch on impact.