Suppose you see the color red. What exact happens in your mental state. Maybe, to understand this, we may need to look into many fields such as Neuro-biology, science of perception etc. Now supposing you concentrate on this feeling of looking at red strongly, you can augment and enhance the feeling (neurological activity increase shown in the diagram). Maybe consuming a hallucinogen drug would do this w/o any effort from your side. So understanding the mental state change and holding on to it to understand it may be a part of meditation.