Ralston told reporters in Grand Junction, Colo., that he initially tried to amputate his forearm three days after becoming trapped on April 26 in a remote canyon in Utah. But he could not pierce the skin with the dull pocketknife.

The next day, his food and water running low, Ralston, 27, did a dress rehearsal of the amputation. He went through the motions of applying a tourniquet, laid out bike shorts to absorb blood and got ready for a quick escape. He worked out how to get through the bone with the ''multi-tool''-type knife he carried, made duller by futile attempts to chip away at the rock that was pinning him. ''Basically, I got my surgical table ready,'' he said.

On the fifth day, he went to work. He broke the radius bone, which connects the elbow to the thumb. Within a few minutes he cracked the ulna, the bone on the other side of the forearm.

''From there, I had the knife out and applied the tourniquet and went to task. It was a process that took about an hour,'' the Aspen resident said, his right arm in a sling. ''I'm not sure how I handled it. I felt pain and I coped with it. I moved on.''

Eric Weiss, a Stanford University Medical Center professor and specialist in emergency wilderness medicine, called Ralston's procedure ''the quintessential example of someone improvising in a dire situation,'' Weiss said. ''He took a small knife and was able to amputate his arm in such a way that he did not bleed to death.''

Slim and pale with short reddish-brown hair, Ralston made frequent references to prayer and spirituality in his news conference. ''I may never fully understand the spiritual aspects of what I experienced, but I will try. The source of the power I felt was the thoughts and prayers of many people, most of whom I will never know.''

Ralston's ordeal began during what was supposed to be a day trip near Canyonlands National Park. He became pinned when a boulder rolled as he climbed over it and trapped his right arm against a cliff face. The boulder weighed an estimated 800 pounds.

Ralston sacrificed his arm after going through most of his water and food. Afterward, he crawled through a narrow, winding canyon, rappelled down a 60-foot cliff and walked approximately 6 miles down the southeastern Utah canyon.