It is hard to know without properly defining all the terms in your question and possibly a sketch to go with.
However, I should imagine it is due to Faraday's law of induction. The rate of change of magnetic flux in the electromagnetic braking system will then be proportional to the velocity of the carriage (I'm assuming this is an elevator question as you have $g$ the acceleration due to gravity.), this will provide a braking force which decelerates the carriage.