Controls whether or not child tweens/timelines are repositioned automatically (changing their startTime) in order to maintain smooth playback when properties are changed on-the-fly. For example, imagine that the timeline's playhead is on a child tween that is 75% complete, moving mc.x from 0 to 100 and then that tween's reverse() method is called. If smoothChildTiming is false (the default except for the root timelines), the tween would flip in place, keeping its startTime consistent. Therefore the playhead of the timeline would now be at the tween's 25% completion point instead of 75%. Remember, the timeline's playhead position and direction are unaffected by child tween/timeline changes. mc.x would jump from 75 to 25, but the tween's position in the timeline would remain consistent. However, if smoothChildTiming is true, that child tween's startTime would be adjusted so that the timeline's playhead intersects with the same spot on the tween (75% complete) as it had immediately before reverse() was called, thus playback appears perfectly smooth. mc.x would still be 75 and it would continue from there as the playhead moves on, but since the tween is reversed now mc.x will travel back towards 0 instead of 100. Ultimately it's a decision between prioritizing smooth on-the-fly playback (true) or consistent position(s) of child tweens/timelines (false).
Some examples of on-the-fly changes to child tweens/timelines that could cause their startTime to change when smoothChildTiming is true are: reversed, timeScale, progress, totalProgress, time, totalTime, delay, pause, resume, duration, and totalDuration.