DHYAN, CONTEMPLATION, is the uninterrupted flow of the mind to the object. This will happen after dharana. So the 1st step i.e. 6th limb - concentration when we have dropped the crowd of objects and chose one object. Once this is done now a person can retain one object in consciousness. Now the second step uninterrupted flow of consciousness towards the object. As if light is falling from a bulb, uninterrupted.
Dhyan, contemplation, mean the consciousness falling on the object in continuity, with no break -- because each break means a distraction. If you can attain the first, the second is not difficult. Remember 1st step is concentration, then comes contemplation so essentially if you cannot attain the first, the second is impossible. Once you drop objects, you choose one object, then you drop all loopholes in your consciousness, all distractions in your consciousness, you simply pour yourself on one object.
Both steps are essential to any work we do. But remember, this is not the end. There is one last step, which is SAMADHI.