Sync Groups keep related animations synchronized with each other, even if they have different lengths. For example, you may have a walk cycle and a run cycle that you would like to blend together so that the character can smoothly accelerate or decelerate. However, the walk cycle for a given character is usually significantly longer than the run cycle? In such a case, directly blending from one to the other would have unnatural results, like an unsightly "beat" as the foot animation switches.
Sync Groups solve this problem by designating one primary animation node as the Leader, and scaling the lengths of all related animations to match the Leader's length. Typically, the Leader is the node with the greatest blend weight. As the weight blends and the follower's weight exceeds the Leader's, the follower becomes the Leader. In this way, the two animations can work smoothly together, offering a seamless transition from one motion to the next.
It should be noted, however, that since the time of the animations is shifting, there are certain animation considerations to take into account. For instance, in the example of blending between walk and run cycles, both animations should begin and end on the same foot. Establishing these standards early will help everything blend much more smoothly!