Decompression valve
A small button-operated valve that vents part of the cylinder pressure during starting, making the engine much easier to pull over. It closes automatically as soon as the engine fires.
Decompression valves are normally found on larger-displacement saws, where compression is high enough that starting without one is genuinely hard on the shoulder and on the starter cord. Press the button before each start; forgetting it simply means a heavier pull, not damage. If a saw becomes unusually easy to start and then lacks power or will not hold revs, a decompression valve stuck open is a plausible cause and a cheap part to replace. Do not confuse the valve with the primer bulb or the choke, which do entirely different jobs.