dc42:
The key bit is that the induced emf always opposes the increase or decrease of current in the coil. As Faraday's Law says, "The induced electromotive force in any closed circuit is equal to the negative of the time rate of change of the magnetic flux through the circuit." Think of an inductor as a device that likes to have a constant (or zero) current flowing through it, and that argues with you whenever you try to change that current.
but why does the additional current flows with the main coil current when the main coil current decreases?Still why it doesn't flow against the main coil current like the one when the main coil current is increasing?