Ensure selected keyframes’ handles have equal length.

Side
Side of the keyframes’ Bézier handles to affect
Left
Equalize selected keyframes’ left handles.
Right
Equalize selected keyframes’ right handles.
Both
Equalize both of a keyframe’s handles.
Handle Length
Length to make selected keyframes’ Bézier handles.
Flatten
Make the values of the selected keyframes’ handles the same as their respective keyframes.