Here an example of the relevance of understanding the proper foundations upon which the mind may function. Besides, this example also illustrates the meaning of the Jhāna factors in actual meditation practice.
As you focus on your in-breath and out-breath, gradually your brain becomes clear. Thus you begin to notice that inside your body some process of purification has begun. And becoming aware of that process, slowly you decide that you should start to follow that trail of purification from your breath into your body. As you do so, feeling into your body, you more and more often reach to the bottom of it. And as you reach the bottom, you find as a natural task opening up before you, to fiddle as it were, your mind into your spine. This, you realise, is the only way to a natural sustainment of your mind in your body.
As you succeed with that, immediately you feel much more alive. Now you notice that you can be aware of your body as a whole. Trying to maintain this condition, you feel that your energy naturally centres around your solar plexus, the point just above your navel. Staying there with your mind, feelings of power fill your body and mind…which will be the beginning of the third Jhāna factor (pitī -joy or exhilaration). Thus, swapping back and forth between your body as a whole and its centre of power just above the navel, you find, that your energy becomes ever more purified.
Feeling this sense of exhilaration, a yet other process gets triggered. As there is now no more need for your heart to pump strenuously blood somewhere else, you feel that your heart remains for prolonged periods an open pool of energised blood or energy.
And as your consciousness has remained aware of this process, alongside with the purification of the bodily energy, consciousness too became subtle and refined….And consciousness being subtle and refined, it becomes cognisant, that, as the vigour of your body increases, some of your bodily energy starts reaching further and further beyond your bodily frame.
Now you again find yourself moving back and forth between two quite different foundations. One is your heart, or more accurately the beat of your heart. The other is the aura or energy field emanating from your body, …having as its centre the heart, but reaching ever further beyond your physical form.
As you succeed in taking this energy field for more prolonged periods of time as the foundation for your consciousness, you become more and more capable to balance your consciousness solely by means of acts of expanding and contracting that energy field. Thus you learn to expand that foundation for your consciousness over ever increasing, at times vast, distances. This then becomes sukkha (happiness), the third Jhāna factor, which, as it stabilises will gradually do away with the prior Jhāna factors.
But then, seemingly even without your noticing, the centre for the functioning of your consciousness moves yet higher. As you feel very very good, your mind, that is, your imagination starts to grow more and more vivid. Thus you begin to see within your sphere of consciousness apparitions which are at first self-evidently for the most part self-created. But the experience of which motivates you to set to work, making experiments, as to how to gain more clear and more objectively real perceptions. Thus, you learn to bring the one-pointedness of your consciousness and the clarity of your mindfulness*, the last two Jhāna factors, gradually to ever greater perfection.
And as a yet higher base for your mind and consciousness becomes more and more involved, your faculty of knowledge too will involve itself in those perceptions. Thus, whatever knowledge you might have acquired before, regarding those fine-material perceptions that you have, will give you insight into life and laws and perhaps the workings of the entire cosmos.
Strictly speaking the last two Jhāna factors are ‘upekkhā’ and ‘ekaggatā’, which I usually translate as balance and one-pointedness, but the meaning should be clear here to be equal.
Note: I made a small animation about the factors of Jhāna some time ago. For further interest in the subject, one may click here. Also, my post on Jhāna Paccaya may be of interest.