The Frontal plane essentially is the plane of movement that whilst facing a wall you bend side to side so that one hand ends up closer to the knee whilst the other ends higher up.
Balance about the frontal plane is crucial given the kind of movements involved in Crossfit’s Constantly varied, functional fitness, at high intensity. whilst most of the movements in Crossfit occur in the sagittal plane, most athletes have a dominant side, this then can create imbalances with the dominant side compensating for the weaker side, over time, this can directly affect Midline stabilisation which can result is sever spinal injuries. What it boils down to is that non-deliberate loss of Balance about the frontal plane ultimately means a loss of movement efficiency and an increase risk of injury.
Focusing on how to improve Balance about the frontal plane, one can refer back to the concept of increasing Work Capacity across broad time and modal domains though ensuring that movements are refined via threshold training thought the process of Mechanics, consistency, Intensity. So you focus on unilateral movements such as lunges and in particular single kettlebell and dumbbell which essentially force the athlete to counter balance the movements and develop that Balance about the frontal plane. As the athlete adopts at the lower weight, intensity and technique, the coach can begin to increase the challenge by dialing up the weight, shortening the time, increasing the volume or even adding in tempo training to continually make a movement more and more challenging.
Almost all movements have some unilateral equivalent that can benefit improving Balance about the frontal plane, such as dumbbell thrusters or single hand kettlebell swings.