The small-hand mechanism is designed for players with either small hands or thin fingers. The 4th finger holes A/D for the left hand as well as the 4th finger right hand ring is covered by a mechanism with a closed key/plateau key. This enables the player to play these notes without needing to cover the tone holes precisely. It is enough if the covered key is touched at any place. This makes obscuring hand positions obsolete and players with tiny fingers can cover tone holes securely.
This modification can be done to any instrument. For schools and bands it might be a good idea to dedicate some old instruments to be altered so very young players can start on these instruments and can later transition to unaltered clarinets when their hands are large enough.