Main toolbar > Curve Editor (Open) > Select a rotation track in the Track View hierarchy. > Track View menu bar > Controller
menu > Assign > Smooth Rotation
Graph Editors > Track View – Curve Editor > Select a rotation track in the Track View hierarchy. > Track View menu bar > Controller
menu > Assign > Smooth Rotation
Animation menu > Rotation controllers > Smooth

Use Smooth Rotation when you want rotation to have a smooth and natural look. Smooth rotation functions the same as Linear Rotation, but uses non-adjustable curved interpolation, and has the following characteristics: