100% pitch is the 'fine' setting for that A/C (aircraft) and 0% is minimum 'coarse' pitch specified A/C adjustment limits.
You want to reduce as much as you want to accelerate faster and same time this reduces airflow pushing the prop/engine to overspeed damage. For climb, increase pitch closer to 100%.
Think of prop pitcj like a manually selectable automatic gearbox - 100%=lowest gear, 0%=highest gear. But remember you can be at the wrong altitude and speed for the wrong gear, and that is also ignoring the differences in engines and aircraft - some had different methods of adjusting/controlling engine revs or prop revs, let alone the auto systems of Daimler Benz verses BMW's version...
Get it wrong with engine overheat and realistic damage on, and you motor if overspeeded, will/can die a mechanical death - dependant on how long it takes to bring it under control that is - sometimes you can hear the 'lumpiness' of damaged engine (akin to fractured bearings/bent valves/stress cracked casings), other times, it'll just stop as if you let/made it rip its valve heads off the valve stems.
Overheating can have a slower death.. Ingame online air-racing can teach you that...