-webkit-animation-timing-function

The CSS   Apple extension property defines how an animation progresses between keyframes. The timing function is specified using a cubic Bezier curve. Use the constants to specify preset points of the curve or the  function to specify your own points. See Visual Effects Timing Functions for additional information about timing functions.

This property applies between keyframes, not over the entire animation. For example, for an  timing function, an animation eases in at the start of the keyframe and eases out at the end of the keyframe. A  defined within a keyframe block applies to that keyframe; otherwise, the timing function specified for the animation is used.