Zero is a philosophy more than a technique but it works as such all the same. The creator of Zero is a martial artist who feared what he would become if he gained more power after seeing how it has corrupted many of the Rankers throughout the Tower. So he focused on the power he already had and honed that to prevent himself from gaining any new power. This philosophy of training massively enhanced his power to a level that left him considered one of the greatest martial artists in the entire Tower.
Any technique can become a Zero technique by stripping away any experience using it and redefining what that experience means when you start from the foundation of the technique. This stripping away actually weakens the power of a technique at first, but increases the depth of the technique and greatly deepens your understanding of it. When that technique is brought back to the level it was before by training it all over again, it becomes considerably more powerful than before.
This is the wisdom of a master giving you the ability to look at every technique you have, no matter the power, and find any and all faults in it. Normally, it takes the aid of someone else to find flaws in a fighting style but by using Zero you can do this automatically by relearning that technique from the position of a master. All techniques and powers will grow stronger through this technique but how exactly that works is up to you.