This article, Wood Release: Great Forest Technique, contains content taken directly from our mother site. All credit of such content is reserved the original authors over at Narutopedia, the original article can be read here: "Wood Release: Great Forest Technique".
A technique that changes one's own arm into big trees. Using chakra, the tissues are transformed into trees at the cellular level. Then, by activating the trees, stimulating them to grow rapidly, they lengthen at high speed and fork into many branches. It is possible to capture the enemy with these branches and at the same time, if one changes the ends into sharp stakes, turn them into countless, sharp, spear-like piercing weapons.
Yamato also used this technique to infiltrate Orochimaru's hideout. After finding a small crack in the wall, the wood squeezed into the hole and grew into a square shape which increased the size of the hole so that he and his team could pass through.