Hi Tensor,
Yes, the two are very similar in their effect, so can be easily confused. When “favouring the mantra as silence”, there is silence with a mere intention (to favour the mantra). When “favouring the intention to remain in silence” there is silence, with a mere intention (to favour silence).
So, if they seem so apparently similar on the surface, then someone may ask, why would one be correct procedure and the other not? One easy way to think about it is to ask the question: “What happens if the mind begins to come out of samadhi into thoughts and memories and attachments?” If someone has switched to favouring the intention to be in silence, then they will have a problem. The mind will be turbulent with a lot of distractions and they will no longer have a mantra to come back to as an anchor. Having the intention to favour silence is not so easy to do when there is no silence in the mind. Of course they could switch back to favouring the mantra, but they they would need to decide at some point that there is not enough silence in their mind to be favouring silence any more, and then make the switch. So, they would need to be engaging in mental analytics to the degree of deciding if sufficient silence is present in the mind, or not.
In this scenario, someone would go from easily favouring the mantra, to easily favouring the mere intention of the mantra as silence, to easily favouring the mere intention of resting in stillness, to mental analytics (asking the question: “Is there sufficient stillness in the mind or not?”), to favouring the mantra again. So, the simple procedure of Deep Meditation would be gone, and replaced with a potentially complicated procedure involving making decisions about the level of silence and stillness present in the mind during meditation.
So, this is why we always favour the mantra, even if the mantra has refined to stillness. It means there is always only one thing to do, and never any need to be analysing the depth of silence, which would divide the mind. Favouring the mantra with our attention over anything else that is going on, includes favouring the mantra with our attention over having the intention to abide in stillness.
As for working out if you are “off” the mantra, or “on” the mantra, that is easy. If you are not easily favouring the mantra with your attention then you are off it, and need to come back to it. If you are easily favouring the mantra with your attention then you are “on” it. If the mantra refines to stillness and you have the mere intention to favour the mantra as stillness, then you are “on” it. The “mere intention” to favour the mantra counts as being “on” the mantra.
It is useful to remember that the goal of meditation is not to rest in stillness and silence. The goal of meditation is liberation (moksha), unity (kaivalya) and Divine love. And as Sey mentioned, that has to do with the cultivation of the witness. The witness is cultivated more effectively with a simple meditation procedure than with a complex one. This is because the witness involves a “stepping back” from the mind, not more involvement with it. Cultivating the witness also involves letting go of attachments. This includes attachments to bliss states and attachments to states where the mind is silent. Attachments can arise in very subtle ways, and the mind can be very clever about how it allows subtle states of attachment to arise, and be maintained. Sticking to the simple procedure of always easily favouring the mantra ensures that these subtle and more refined attachments will also fall away if they arise. Ultimately yoga is about being at peace regardless of what is happening in the body and mind.
I recently wrote a blog on the difference between states of consciousness, such as the mind being silent (samadhi), and the process of liberation. That is here:
Is enlightenment a state of consciousness?