Mu Training

Mu Training is a special form of meditative martial arts training which Poo undergoes in EarthBound. It involves the ability to ignore everything around you. The training takes place on a small, mountain-like peak in the southeast of Dalaam. Upon starting the training, Poo is besieged by illusions and distractions, including one which tells him his master is calling for him. If Poo succumbs to distraction, then he fails the training and must start over.
However, if Poo focuses and remains still, he will eventually enter an illusionary world, where an enigmatic being repeatedly asks him if he minds if he destroys varying parts of him, ranging from arms and legs to his very mind. Answering "Yes" will result in Poo losing a portion of his HP. If he manages to keep meditating despite all of this, he completes his training and seeks out Ness using PSI Teleport β.
Etymology
Mu can mean "not" in both Written Tibetan and Written Burmese. In Chinese and Japanese Mu means: nothing, not, nothingness, un-, is not, has not, not any. In Buddhism this is the key to liberation to letting one's mind become "awakened" by letting go all worldly desires. This is fitting as Poo is meditating and despite everything the spirit of his ancient ancestor did to him he reached Mu and achieved PSI when his training was complete.