If your timing and position are not excellent and your body is not unified. Your waist and legs need more development.
有不得機得勢處，身便散亂，其病必於腰腿求之，上下前後左右皆然。
– The Theory of Taijiquan by Zhang Sanfeng
This rule is easy to overlook. A lot of people underestimate it because it appears to be a simple explanation of the what happens when you don’t do Rule 5 properly.
Yet, it is the single most common error I see.
Any error in the lower body is magnified through everything else you do.
There can be a number of issues here but the most common and fundamental one is:
Not enough leg strength.
Tai Chi is not about strength and this is exactly why you need it.
Tension is a weakness that will prevent the coordination necessary for Tai Chi Rule #5.
If you are stronger, less of your total strength is needed for standing and moving, so more of you can be relaxed.
You need two types of strength. Train both, they will help each other.
Muscular strength
Do squats.
On youtube you can find thousands of variations on the basic squat for every fitness level.
Find one appropriate for you and get to work.
Tendon and ligament strength
The stronger these are, the more you can relax your muscles, and the better your Tai Chi will be.
Squats will help. You can find specific exercise for this on youtube as well.
Use them.
Relaxing for strength drill
Stand in a Tai Chi posture and relax.
Use the Root at the Feet drill to get the feeling of relaxing and letting your body weight sink downwards into and through the legs.
Now relax deeper.
Breath in, relax and sink.
Feel the sensation of sinking inside your body. Your physical stance should only settle slightly.
Breath out, relax and sink even deeper.
As you relax deeper you should feel more and more weight on the legs.
Relax until the weight on your legs is excruciating. Relax deeper.
If the weight on your legs is not excruciating, you are holding too much tension.
Relax your lower back, relax your hips, relax your shoulders, relax your thighs, calves, feet, etc…
Do this while stepping.
The weight crushing down through your legs should be enough that you cannot lift a leg until you transfer all your weight to the other one.
As you step make sure you do not tense and bring some of the load off of your supporting leg.
Do this with your form.
Keep proper leg alignment!
You are putting a lot of stress on your legs.
Do this in short sessions at first. Build into it slowly.
It will take time for your legs to get stronger.
Pay close attention to the proper leg alignment in Tai Chi Rule 5. There should never be any awkward pressure or pain in the knee.