The 70/30 stance
In stance (A) the back leg knee has collapsed and the back foot rolls onto the inside edge on the ball of the foot. This stance is indicative of weak inner groin muscles and frequently leads to a similar fault in 100% stances as the player sits down into their back leg and the knee falls in instead of remaining over the foot. This stance is not only extremely weak, it is also very harmful to the knee and will lead to knee pain.
Stance (B) is a solid and correct 70/30 weight distribution. Both feet are flat on the floor. The heels are shoulder width apart. The forward knee is over the toe both toe to heel and side to side. The back leg is relaxed and engaged. Hips are facing the same direction as the forward foot. This stance optimizes stability and suppleness.
In stance (C) the back leg is too straight and braced. This mistake usually goes along with weight distribution more like 60/40 and hips not properly oriented. This stance is really stuck in the pelvis and the lower back is completely locked up so that the player has no alacrity or ability to neutralize.