The process of desaturation begins when the diver:

Desaturation is the process through which the body gradually releases excess nitrogen absorbed during a dive. This process typically begins when a diver starts ascending toward the surface.

As a diver ascends, the partial pressure of the gases in the surrounding environment decreases, which allows the nitrogen that has been absorbed by the tissues during the dive to start being released back into the bloodstream and expelled from the body. This is crucial because if the diver ascends too quickly, the rapid decrease in pressure can cause nitrogen bubbles to form, leading to decompression sickness, commonly known as "the bends."

When a diver is descending, they are absorbing nitrogen due to the increased pressure, while safety stops and decompression stops are specific measures taken during ascent to allow for controlled desaturation. In these contexts, the diver is already in the process of ascertaining a safe ascent rate and giving their body additional time to eliminate nitrogen safely.

Therefore, beginning the ascent marks the initiation of the desaturation process, as changes in pressure trigger the release of nitrogen from the tissues.