To pin a tree, a hand-powered drill (called an auger) is used to make a hole in a living, standing tree about one inch in diameter and four or five inches deep. Into this hole is placed quartz rock, or a ceramic pin that has been kiln fired to "cone 10" hardness. The hole is then filled with a silicon adhesive, and masked with a piece of tree bark.
Pinning trees to dull mill blades is a form of ecodefense and monkeywrenching.