An unstoppable jutsu, only three have ever and will ever master it, because the only way to share the technique is for someone who already posseses it to annoint someone else. Annointed and taught to Echo and Monk in a realm of pure chakra, Echo and Monk had to prove themselves by having a battle that would produce enough chakra to surpass a Bijuu. Succeding(eventually), they trained in savage methods that would easily kill the weak, and learned the most powerful technique created.
The Chakra Sync can only be accomplished through complete chakra control, element control, and the insane ability to infuse your physical mass with your chakra. Keeping their appearances, their bodies dematerialize, leaving their spiritual energies outlined with their respective chakra colors(green-black, Echo, gold, Monk) In this form, they are pure chakra, and can corrupt or control anyone's chakra as they wish. Meaning they can literally rip someone's flesh out with their own chakra, complete drain their network, or even seal and release Bijuu. This is the ultimate technique, as any type of chakra, spirit, or life energy becomes controllable by the user, making this technique virtually invincible.
A major side effect of this transformation is that after use, the user cannot access their chakra for a month, and if used more than once a month, the user will permanently loose consciousness and be nothing but chakra for the rest of eternity(As seen with The Ancient).