Mindfulness of sensation or contemplation of sensation is known as Vedananupassana Satipatthana. Usually at the beginning of the practice, the meditator feels the unpleasant physical sensations as well as mental sensations. Here we need to explain again the two kinds of sensation:
If the feeling or sensation arises depending on physical processes, it is known as Kayika vedana We may translate it as physical feeling or sensation, or bodily feeling or sensation. If the reeling or sensation arises depending on mental processes, it is called cetasika vedana. We may render it as mental feeling or mental sensation. Actually, every feeling, every sensation is a mental process, not a physical process. However, sometimes feeling or sensation arises depending on the physical process of discomfort. When a meditator feels discomfort in his body, then unpleasant sensation arises. That unpleasant sensation is called k_yika vedan_ because it arises depending on physical processes.
In the beginning of the practice, a meditator generally experiences mostly unpleasant mental and physical sensations. But whatever sensation he may experience, he must observe it very attentively, energetically and precisely so that he can realise the true nature of that feeling or sensation. The specific and the general characteristics or the reeling must be thoroughly realised so that he will not be attached to it or repulsed by it. This is Vedananupassana Satipatthana - mindfulness of feelings or sensations. Whenever feeling arises, it must be observed and noted as it really occurs.
It is natural for a meditator to be afraid of unpleasant physical sensation which he experiences in his meditation practice, but painful sensation is not a process that should be feared. Pain is a natural process that should be thoroughly understood by being aware of it as it really occurs. When a meditator can observe pain successfully with persistent effort, then he can realise its true nature - the specific and general nature of pain. Then the penetrating insight into the true nature of that pain or unpleasant sensation will lead the meditator to the higher stages of insight. Eventually, he could attain enlightenment by means of this painful sensation.