Inbetweens offer a way to adjust animation timing by adding or removing one frame or time unit. Keys and Breakdowns preserve their tangent characteristics, resulting in a minimal change to existing animation curves. See Set Inbetweens.
Using Add Inbetween will cause the animation’s keys to be shifted by one whole time unit forward from the current time.
Using Remove Inbetween will cause the animation’s keys to be shifted by one whole time unit backward from the current time.