The most powerful attack of the yama senken so powerful that it wasn't even written down. it is well known that the human body can focus only that much ki before the fine balance between perfect control and absolute chaos wavers. Though the amount of power that one can channel using one's body varies from person to person the symptoms that manifest when that fine balance is reached or exceeded is well documented by the masters of the olden days. The body would be put under extremely large amounts of stress and when the energy focused is too high, it simply eats away the body, bones and all. Genma Saotome had tried very hard to circumvent this block and he succeeded.
The answer is quite simple really. All you had to do is find another focusing medium and what better medium than the earth itself. The lei-lines that flow throughout the planet can channel an insanely large amount of power. It is in techniques that utilize this 'constructs' that the Yamasenken and the Umisenken specializes in. The entire 'construct' based attacks channel very large amounts of energy and are a danger to everything for miles around should something go wrong. It is for these reasons that the senken was sealed away. The attacks are simply too dangerous to be used in anything but the most desperate of situations. The technique however has its substantial flaws, such as if the user multiplies their base ki too much, the uprise of ki could easily obliterate them, or severely damage their body.