In ACIM there is a lot of talk about imprisoning someone, and also about setting them free. Jesus actually says that you can either a) amplify the sickness of another by reinforcing beliefs they already have, or b) weaken the sickness of another by withdrawing belief in it.
It is one thing to realize that you can potentially "positively" help someone to heal and to set them free, by not projecting guilt or fear or sin onto them, but it is another to willingly look at the possibility that you may be "cursing" them with your ego.
This doesn't mean necessarily that you originate a person's sickness, or that you can curse them against their will. But Jesus does say that you can reinforce beliefs they already have, exploit their weaknesses, and essentially increase their guilt or sickness by projecting yours onto them. We do "affect" each other.
It however remains true that a person who is strong in the truth becomes invulnerable, and has set themselves free both from imprisonment and from the potential for being imprisoned. It is not possible to drag such a person into the darkness.
But if a person has volunteered to "play along with" the idea that they are guilty, or deserve to suffer, or to not be loved, this is like a kind of invitation. If we just lay down and accept that we should suffer as a result of being guilty, we make ourselves sort of "open to" forms of suffering or attack, allowing or using them to hurt ourselves. If we set up a mechanism of self-hurt, it becomes possible for another person to leverage that.
In essence, Jesus is saying that if someone buys into sin and guilt, the projected sin and guilt of others can compound it. It can make their situation worse, further weakening their mind and health. I've certainly heard stories of people falling sick and having medical issues due to being in proximity to guilty, blameful or fearful individuals. And I can say from personal experience that toxic-energy people have to sometimes be avoided.
If it's true that your mind can influence another mind, which appears to be exactly what Jesus is saying - because minds are joined - then it is possible to either abuse this fact or use it for constructive purposes. When the mind is turned to more constructive uses it can leverage the strength it sees in another to amplify that strength, increasing the light and innocence in another's mind, and therefore inducing healing on their behalf.
We need therefore to be mindful that either we are blessing or cursing, hurting or healing. We can pull up or we can pull down. We can be the light of the world or the darkness of the world. And what we choose to give to another we will experience for ourselves. We cannot escape this law because it's the law of God. What you do unto others, you to do yourself, and vice versa.
"Casting spells" merely means "affirming error", and error is lack of love. When man projects this onto others, he DOES imprison them, but only to the extent that he reinforces errors they have already made. This distortion makes them vulnerable to the curse of others, since they have already cursed themselves. The miracle worker can only bless, and this undoes the curse and frees the soul from prison."
"The sick must heal THEMSELVES, for the truth is IN them. But, having OBSCURED it, the light in ANOTHER mind must shine into theirs, because that light IS theirs."
"Through this remembrance you can change THEIR minds about themselves, as I can change YOURS. Your minds are so powerful a light that you can look into theirs and enlighten them, as I can enlighten yours."