Tai chi is balance; physical, mental and energetical. Imagine an old-style balance scale where one side has a pound (or kilo) of metal, and the other side a pound (kilo) of feathers. If you add one feather to the yin side without adding an equal amount of weight to the metal (yang), the scale will be out of balance. This is true in our movements as well.
In tai chi, like the scales, we strive for a balance between yin and yang. However, when players shift their weight, often times that balance is lost. When shifting their weight back, they only think about going back without having anything going forward, creating an imbalance. Conversely, when they go forward, they only think about putting weight into their front leg and neglect the back leg. They think of forward is forward and backwards is backwards, which makes it all yang or all yin.
When going forward it is the front leg that gets the weight and the rear leg that is often neglected. Following the concept of the scales, as you are putting more weight in the front, you must add more to the back for the scales to remain balanced. Therefore, as you shift forward, sink the back foot into the floor with your mind. Your front leg gets 65-70% of the physical weight, so you need to add 65-70% mental weight into the rear leg. When you do that then both legs are equally rooted, and their yin and yang qualities are balanced.
The posture Roll Back for example, is a backward or yin movement. To counterbalance it, there must be a forward, or yang movement. However, many players when they shift their weigh to the back leg, they completely empty their front leg. A better way is to shift the weight back but with your mind sink the front leg into the floor. This way the physical weight going into the back leg is balanced by the energy weight in the front.
These are just two examples. Every movement in tai chi has yin and yang. It is important for you to figure out how to balance them.