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Lanz law states that the direction of the induced emf and hence the induced current in a closed circuit is always such as to oppose the change in magnetic flux producing it.
Therefore, when you try to generate a current by means of a coil in a closed circuit and a magnet, there is an opposing force; the coil gains the ability to oppose the movement of your magnet! As a result, you must apply a force to keep you magnet moving. You certainly know that for a current to be produced, the circuit should be first of all closed and then, your magnet should move in and out of your coil rapidly. Well, you will need more energy to keep the magnet from moving. That energy you supply will give the current produced.
If your circuit is open, no current can be created, so there will be no opposing force. On the overall, you'll need more energy to force the magnet into the coil and out of the coil.
Hope this cleared a few things. Post back if still confused! I'll be able to focus my explanation.
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