Replacing forward bend w/ side bend is a characteristic we focus on often and has a strong relationship to talent

From a recent @dan_aucoin13 analysis, pro's exhibit this trait ~87% of swings, while amateurs do ~70% of swings

Here's what it looks like + mini thread below

Once the swing starts, forward bend is lost as a result of hip extension (think finishing off a squat) but in order for the hitter to not completely come off the ball, side bend must be added that is >= the value of forward bend at first move.

On the graph, we can see forward bend (green line) is set for the height of the pitch just after first move

As the hitter is turning, side bend (blue line) is rapidly added in order to maintain spine angle.