"Please speak about the line in the course that goes – if you’ve practiced rightly, you’ll know because you shouldn’t feel the body at all"
This is from the section "sickness is a defense against the truth", which goes like this:
"Healing will flash across your open mind, as peace and truth arise to take the place of war and vain imaginings. There will be no dark corners sickness can conceal, and keep defended from the light of truth. There will be no dim figures from your dreams, nor their obscure and meaningless pursuits with double purposes insanely sought, remaining in your mind. It will be healed of all the sickly wishes that it tried to authorize the body to obey.
Now is the body healed, because the source of sickness has been opened to relief. And you will recognize you practiced well by this: The body should not feel at all. If you have been successful, there will be no sense of feeling ill or feeling well, of pain or pleasure. No response at all is in the mind to what the body does. Its usefulness remains and nothing more."
The "practice" it is referring to is spoken about earlier in the section, relating to accepting the Truth in you rather than putting up defenses against the truth. For example "Truth has a power far beyond defense, for no illusions can remain where truth has been allowed to enter. And it comes to any mind that would lay down its arms, and cease to play with folly".
Jesus says that if you are successful in accepting/admitting the truth, and dwelling in that truth, then there will be no sense of feeling ill or feeling well, or in pain or having pleasure. He says the mind would in such a state not "respond" to what the body does, in other words, it will not react from ego. The mind will see that the body is just there to serve it and not be distracted by anything the body appears to do or not do. This is a mind that knows that it is in charge and causes the body, rather than the other way around.
See, what happens is, when we project causality and power and responsibility and identity outside of ourselves, it then seems to "do what it does" externally, and "to" us, seemingly against our will. As the mind believes that now some illusory source outside of it is attacking it and causing it, this produces symptoms of sickness. Sickness is external seeking for the truth, ie you believe there is truth outside of you which is causing you. Truth is always equated with causality.
Seeing the external as having power over you, as being a cause, the mind cannot help but perceive that this external power is capable of changing you. Or in other words, that it can produce "effects" in you, against your will. The belief in external causes automatically puts the mind into a certain mode of perceiving where it both perceives that some "other" is causing it, and simultaneously that the mind is now determined by, decided by, affected by, changed by, whatever that cause chooses on your behalf.
The disowning of responsibility for decision making makes you believe that someone or something else is choosing for you, against any remaining will that you still own, and that it truly has the power to change who you are. Literally, this belief, leading to mistaken perception, makes it seem as though your identity can be altered, and the "meaning" of what you are, can be altered by an alien will outside of you. Thus you are victim of it and can do nothing to stop it.
This is really the mind victimizing itself by seeing itself in this way. It is the mind's own choice to disown its own power, to project its own attack, to deny its own identity etc, and this creates a split mind. The mind literally invents and becomes its own attacker. And the mind will also put into place any "effects" that are supposedly going to come from that separate self, creating the illusion that separate selves cause effects that you do not ask for, yet you invented both of these.
The belief in an external power or cause therefore automatically makes the mind "adjust itself", ie adjust your idea of your Self, causing "changes" in what you are. These changes are effects, caused by the dream world, apparently. These effects are your inner-self's attempt to adjust to "match" or correlate to what the external cause is saying you should BE, based on its decision about you. This produces within what you would call a REACTION.
In the hierarchy of illusions, depending on what the external cause seems to be and how much you believe in it, it will "call for" different kinds of reactions. If someone swears at you and calls you bad names, and if you believe there is some truth to that having given power to that person, then your mind WILL generate symptoms or effects of "hurt" or "anger" etc, as what you think of as "reactions". They are really just automatic, unconscious counterparts to the belief you have in how the external is defining you.
In this mode, your mind becomes like a mirror in which you see a reflection of what the external seems to see in you. You reflect back to it what it seems to put into the mirror, and you experience that reflection as your emotional reaction, which then bounces back out. Projection makes perception.
When you practice forgiveness and identify your use of sickness as a defense against truth, and become willing to accept the truth instead of reject it, your mind is no longer going to see power as outside of it, is no longer going to see itself as a victim, is no longer going to believe in someone else having a will over it, and is not going to buy into any external suggestions as to what you are.
Instead it is going to remain calm and still and sure of its identity. Sure of the truth about itself. It is not going to enter into a split mind and it's not going to attack itself. It's not going to defend against truth or attempt to get rid of truth. It won't try to displace truth onto an external "attacker" and won't therefore have to experience the "reactions" which accompany that projection of causality. Instead the mind will just see the body as not a cause, and will not see a need to react to it, and instead will see it only as a tool for the mind's direct outward expression.
As such, when you are in this state and are in the truth, you will have withdrawn from the body all of the projections of power and causality that "made it seem real" before. All of the ways that sickness showed up in the body was simply because you believed in it and gave reality to it. You made it a thing. And now you have withdraw that making and returned your mind to truth.
This lifts off of the body all of that heavy illusion, the defense against truth, which lightens your awareness of it. It no longer seems so real, no longer seems to bother you, no longer seems to cause you. I am presuming that Jesus is saying if you do this quite strongly you will actually become very detached from the body almost as if the body is not even really there. Your lack of belief in it, your lack of thinking it has power over you, shows you it is just an illusion, like a thin veil, and is barely even noticeable.
This makes me think also of the miracle principle which says "Miracles transcend the body. They are sudden shifts into invisibility, away from the bodily level. That is why they heal." Miracles make illusions disappear, and ultimately will make the body disappear. As the mind accepts the truth, ie accepts the atonement, it overlooks the body and the body will barely "register" as existing at all.
Overall I think that this "practice" may yield the results Jesus speaks of when it becomes more advanced. It seems to involve accessing the atonement i.e. the Truth, so would be part of a forgiveness process. Yet He does refer to it as practice so, we should practice this to experientially discover the state of awareness He is speaking of.
I'm not quite certain that I've explained this correctly but I feel I'm close, so let me know if you still have some questions about this.