This is a process called electrochemical machining it's basically electroplating in reverse you apply a current flowing in the right direction and with a flow of salt water to provide the circuit you bring the tin close to (but not touching) the steel the current then eats away at the steel etching the shape of the tin into it. Since there is no contact the tin can be pretty delicate and in industrial settings it can produce results you just can't get any other way.
This video shows a home brew version