Slow in, slow out is a principle of animation.
From 2:14 in the video above, an example of slow in, slow out is given. It describes the movement of objects and the effects of gravity and natural movement. It applies acceleration and deceleration to an object as it rises and falls. In the specific video examples above, as the pendulum-like object swings, it accelerates towards the centers and decelerates from the center to the top as it swings back around. If Slow in, Slow out wasn’t applied here, it’d move in consistent speeds and wouldn’t look as natural and perhaps as fluid as with the principle.
at 0.26 above (this is the same example for arc), in the arc scene there’s also slow in slow out as the character swings its heads.