The alternating current flows in the coil and produces inductive effects in the aluminium ring.
A rising current creates voltage that opposes the inducing current. A decrease in current produce voltage that reinforce the inducing current.
The resistance of the aluminium ring is very low thus the electric induced current is very strong.
As result of the reaction the ring is taken away.
The statement that each kind of opposition occurs is called "Lenz's law".

If is placed on the iron core when the current is already flowing, the aluminium ring floats on the magnetic field. The causes are the same as before.
The aluminium ring stays in the position that causes the smaller induced current.