I am not sure if there is answer for my question but my English is not good enough to search for it.
The problem:
I would like to have stepper motor connected to the gear that pulls string but I would like to make it also manually movable so my question is how to disconnect or make mechanism neutral when motor is not working? This must work in both directions.
wiktoryoko:
But is there any mechanism which only engages motor when it needs? (not electrically)
Look for an electric clutch mechanism. They used to be and maybe still are on automobile air conditioning compressors. They use electromagnets to pull clutch plates together. Likely to be too large for your project, but there must be lots of others.
Paul