For some reason 100% weight shift is one of the hardest things to teach. It seems a large proportion of people, when asked to shift ALL their weight onto one leg, cannot lift the other leg off the ground without re adjusting or pushing off it .... which means they are not actually fully shifted over.
Think about it, if you cannot lift your leg off the ground without re aligning, you cannot be 100% on the other side. To test, you should be able to pick your leg up, put it down, pick it up again without the rest of your body moving - no shifting left or right, or front or back, or wobbling. Look in a mirror to check.
An observer looking at the upper half of your body only, should not be able to tell when you pick your leg off the ground - the upper body should look the same, as though nothing is happening. If you can do that, then you have found your 100% and the body alignment needed to make it work.
Now do it moving, as you walk, as you pendulum, as you flow.
Now, add to this the ability to balance and catch your weight so the whole shifting is controlled and you now have better options for stealing range and escaping without telegraphing, and the ability to hold neutral longer. You can also control the timing better and spend less time in range of your opponent. Oh, and as you can contain your power better, you lessen your chance of 'running out of angle' and increase your options for suddenly switching directions, changing tempo or dropping your weight for power when you need to.
And did I mention this means you can kick too ..... ?
Here is Jay Pugao, another of Sonny's students and a great exponent of the 100% weight shift.